Vape liquid contains four main ingredients: a base of propylene glycol and vegetable glycerin, nicotine, and flavoring chemicals. But what you actually inhale is more complex than what’s in the bottle. When the liquid hits the heating coil, it produces an aerosol containing dozens of additional compounds, including aldehydes, volatile organic compounds, free radicals, and trace metals. Flavored e-cigarette aerosols contain between 94 and 139 detectable compounds, far fewer than the 6,500-plus found in cigarette smoke, but far from harmless.
The Liquid Base: Propylene Glycol and Vegetable Glycerin
Every e-liquid starts with a base of two clear liquids: propylene glycol (PG) and vegetable glycerin (VG). These are the substances that produce the visible vapor cloud when heated. PG is thin and watery, carries nicotine efficiently, and delivers a stronger throat hit. VG is thicker, slightly sweet, and produces much denser clouds. Most e-liquids use some blend of both, though disposable vapes sold at convenience stores often use pure PG.
The ratio matters for the vaping experience. A 50/50 blend is common in starter devices. People who want bigger clouds lean toward higher VG mixes, while those who want a sensation closer to smoking cigarettes prefer more PG. Neither substance is toxic to swallow in small amounts (both are used in food products), but inhaling them as a heated aerosol is a different exposure route, and that’s where other chemicals enter the picture.
Nicotine Types and Concentrations
Most vape liquids contain nicotine in one of two forms: freebase nicotine or nicotine salts. Freebase nicotine is harsher at high concentrations, so it’s typically used at lower levels. Nicotine salts are smoother to inhale, which allows manufacturers to pack much higher doses into small devices. Products commonly come in 1% or 5% nicotine by weight. A 5% nicotine salt pod delivers roughly 50 mg/mL, comparable to the nicotine exposure from a pack of cigarettes spread across the pod’s lifespan.
Nicotine-free e-liquids do exist, but they represent a small share of the market. If a product doesn’t clearly state 0% nicotine, assume it contains some.
Flavoring Chemicals
Flavoring is where the ingredient list gets complicated. E-liquids use food-grade flavoring compounds to create everything from mango to crème brûlée. The problem is that “food-grade” means safe to eat, not necessarily safe to inhale. Three flavoring compounds have drawn particular concern: diacetyl, acetyl propionyl, and acetoin. Diacetyl gives a buttery or creamy taste and has been linked to serious respiratory disease in workers who inhaled it in popcorn factories. Acetyl propionyl is a closely related compound with similar concerns. Acetoin was introduced as a safer substitute, but research has shown it can convert into diacetyl when heated. All three remain present in commercially available e-liquids, though some manufacturers have voluntarily removed them.
Flavored e-liquids also contribute to overall chemical complexity. Unflavored aerosols contain roughly 72 to 79 detectable compounds, while flavored versions contain 94 to 139. The additional compounds come almost entirely from the flavoring ingredients and their breakdown products.
Chemicals Created by Heating
Some of the most concerning substances in vape aerosol aren’t in the bottle at all. They form when the liquid hits the heating coil. Propylene glycol breaks down into formaldehyde and acetaldehyde. Glycerin produces acrolein, a potent lung irritant. All three of these aldehydes are also found in cigarette smoke and contribute to cardiovascular and lung disease risk.
In lab testing, 10 puffs from an e-cigarette produced between 8 and 40 micrograms of formaldehyde depending on the liquid, compared to about 74 micrograms from a single conventional cigarette. Acrolein levels ranged from 1.6 to 5.8 micrograms per 10 puffs versus 120 micrograms from a cigarette. So the exposure is lower, but it’s not zero.
Power output matters enormously. At higher wattage settings, formaldehyde production skyrockets. One study found that increasing power from 9.1 watts to 11.7 watts pushed formaldehyde from about 20 micrograms to nearly 130 micrograms per 10 puffs. At 16.6 watts, acetaldehyde levels jumped to over 530 micrograms. Using a device at high power settings, or “dry hitting” when the wick runs low, dramatically increases exposure to these toxic byproducts.
Heavy Metals From the Coil
The heating coil itself contributes metals to the aerosol. Because the liquid sits in direct contact with a metal coil and metal alloys, those metals leach into what you inhale. Researchers at Johns Hopkins University found nickel, chromium, lead, and arsenic in e-cigarette aerosol. Nickel and chromium are both established inhalation carcinogens, meaning they can cause cancer when breathed into the lungs repeatedly.
What surprised researchers is that some toxic metals, including arsenic and lead, were present in the e-liquid even before it touched the coil. This suggests contamination during manufacturing, not just coil degradation. The metals were then found at higher concentrations in the aerosol after heating, meaning the coil adds to an already-present problem.
Volatile Organic Compounds and Free Radicals
Vape aerosol contains volatile organic compounds (VOCs) including benzene, toluene, and ethylbenzene. Benzene is a known carcinogen. In tested e-liquids, benzene concentrations ranged from about 0.8 to 5.1 micrograms per gram of liquid, and aerosol levels reached up to 4.2 micrograms per 20 puffs. These levels are low compared to cigarette smoke, but benzene has no safe threshold for cancer risk.
E-cigarette aerosol also contains reactive oxygen species, which are highly reactive molecules that damage cells. These include hydroxyl radicals, superoxide, and hydrogen peroxide. Researchers detected between 1.2 and 8.9 nanomoles of reactive oxygen species per puff, with hydrogen peroxide accounting for 12 to 68 percent of the total. Higher voltage settings produced more of these damaging molecules. This kind of oxidative stress is one mechanism through which vaping may injure lung tissue over time.
Contaminants in Illicit Products
Black-market vape cartridges, particularly those containing THC, can include adulterants not found in regulated nicotine products. The most notorious is vitamin E acetate, an oily substance used as a cheap thickener in illicit THC cartridges. It was identified as the primary cause of the 2019 EVALI outbreak, which hospitalized thousands of people with severe lung injury. In Minnesota, 92% of EVALI patients had used THC products containing vitamin E acetate. It was found in brands like Dank Vapes and various counterfeit cartridges.
Vitamin E acetate was not detected in products seized before 2018, suggesting it was introduced as a cost-cutting filler around that time. While the EVALI crisis brought public attention to the issue, unregulated THC cartridges remain available, and their contents are unpredictable. This risk applies specifically to black-market cannabis products, not to commercially manufactured nicotine e-liquids sold through licensed retailers.
How Vape Aerosol Compares to Cigarette Smoke
Cigarette smoke contains over 6,500 identified chemical compounds. Flavored e-cigarette aerosol contains roughly 94 to 139, and unflavored aerosol contains 72 to 79. That’s one to two orders of magnitude less complex. The concentrations of harmful compounds are also significantly lower across the board. Formaldehyde, acrolein, and acetaldehyde are all present in vape aerosol, but at fractions of what a cigarette produces under normal use conditions.
This doesn’t make vaping safe in absolute terms. It means the chemical exposure profile is different and, by most measures, less intense than combustible tobacco. But the aerosol still delivers carcinogens like benzene, heavy metals like lead and nickel, reactive molecules that stress lung cells, and flavoring compounds with unresolved inhalation risks. For someone who has never smoked, these exposures represent a net addition of harm rather than a reduction.