When you vape, you inhale an aerosol containing far more than just flavored water vapor. The mist pulled into your lungs is a mix of propylene glycol, vegetable glycerin, nicotine, flavoring chemicals, toxic byproducts created by heating, trace metals from the coil, and ultrafine particles small enough to reach the deepest tissue in your lungs.
The Base Liquids: Propylene Glycol and Vegetable Glycerin
Every e-liquid starts with two carrier liquids: propylene glycol (PG) and vegetable glycerin (VG). These are the substances that produce the visible cloud and carry the nicotine and flavoring into your airways. Commercial e-liquids typically range from a 70/30 PG/VG ratio to nearly pure VG, with 50/50 blends being common. Higher PG concentrations produce a stronger “throat hit,” the sharp sensation at the back of the throat that mimics smoking. Higher VG ratios create thicker, smoother clouds.
Both PG and VG are classified as safe for ingestion in food, but inhaling them is a different matter. When heated by the coil, these liquids don’t just evaporate cleanly. They break down into smaller, potentially harmful chemicals, which we’ll get to below.
Nicotine Levels Vary Widely
Most e-liquids contain nicotine, though concentrations differ depending on the formula. Traditional freebase nicotine liquids typically go up to about 18 mg/mL. Nicotine salt formulas, the type used in many pod-style devices, are available up to 20 mg/mL or higher in some markets, yet feel smoother on the throat. This smoothness makes it easier to inhale higher doses without discomfort, which is one reason nicotine salt devices have been linked to faster dependence in new users.
Flavoring Chemicals and Lung Toxicity
The flavor in your vape comes from dozens of individual chemical compounds, many borrowed from the food industry. At least 65 flavoring ingredients found in e-liquids or their aerosol have been shown to damage cells in the respiratory tract, cardiovascular system, or skin in lab studies. Cinnamaldehyde, the compound that gives cinnamon its taste, is the most frequently reported to be toxic to cells, followed by vanillin, menthol, ethyl maltol, benzaldehyde, and linalool.
These compounds cause harm in several ways: they kill airway cells directly, generate reactive oxygen species (a form of oxidative stress), and impair the natural clearance mechanisms your lungs use to sweep out debris and pathogens. Diacetyl, a buttery flavoring famously linked to “popcorn lung” in factory workers, and its close chemical relative 2,3-pentanedione have also been detected in e-liquids.
An additional concern is that some flavoring chemicals react with propylene glycol inside the liquid itself, forming new compounds called PG acetals. Cinnamaldehyde, vanillin, and benzaldehyde all form these reaction products, which means the chemical you inhale may not even appear on the ingredient label because it was created after mixing.
Toxic Byproducts From Heating
Heating propylene glycol and glycerin doesn’t just turn them into vapor. It also breaks them apart into smaller molecules called aldehydes, several of which are known to be toxic. The three most important are formaldehyde, acetaldehyde, and acrolein. All three are also found in cigarette smoke and contribute to cardiovascular disease and lung damage. Every e-cigarette tested in published research has produced all three of these compounds.
The amounts are generally much lower than in a cigarette. In one study, a conventional cigarette produced about 1,240 micrograms of acetaldehyde, 120 micrograms of acrolein, and 74 micrograms of formaldehyde per 10 puffs. Low-power e-cigarettes in the same study produced less than 1 microgram of each. Higher-power devices with refillable tanks generated considerably more, with some producing 15 to 63 micrograms of acetaldehyde and 8 to 40 micrograms of formaldehyde per 10 puffs.
Power matters enormously here. When researchers increased a single device’s output from about 12 watts to 15 watts, formaldehyde levels tripled from roughly 130 to 387 micrograms per 10 puffs, and acetaldehyde jumped sixfold. This means the same liquid in the same device can produce dramatically different levels of toxic chemicals depending on your wattage setting. “Dry hits,” where the wick isn’t fully saturated, push temperatures even higher and worsen the problem.
Metals From the Heating Coil
The metal coil that heats your e-liquid also sheds particles into the aerosol you breathe. The specific metals depend on the coil material. Nichrome coils, one of the most common types, release nickel and chromium directly into the aerosol during heating. Kanthal coils, another popular option, are made of iron, chromium, and aluminum. Copper and lead have also been detected, though these tend to come from solder joints and other device components rather than the coil itself.
Research confirms that chromium and nickel don’t simply leach into the tank liquid first. They transfer directly from the heated coil into the aerosol you inhale, bypassing the liquid entirely. This means refilling with fresh liquid doesn’t reset your metal exposure. The coil continues to degrade with use.
The relationship between wattage and metal levels is not straightforward. Some studies find significant increases in metal content at higher power settings, while others don’t, suggesting the connection may depend on the specific wattage range and coil type. What is consistent across research is that the metals are present in measurable quantities regardless of power level.
Volatile Organic Compounds
Beyond aldehydes and metals, vape aerosol contains volatile organic compounds (VOCs) including benzene, toluene, ethanol, acetonitrile, and isopropyl alcohol. Benzene is a known carcinogen. Toluene can affect the nervous system. These were detected at low concentrations in e-cigarette aerosol testing, but they represent yet another category of exposure that doesn’t exist in clean air.
Ultrafine Particles That Reach Deep Lung Tissue
The aerosol particles in vape clouds are extraordinarily small, typically between 10 and a few hundred nanometers in diameter, with the peak concentration around 30 to 40 nanometers. For perspective, that’s roughly 1,000 times smaller than the width of a human hair. These qualify as ultrafine particles, and their tiny size is precisely what makes them concerning.
Larger particles tend to get caught in the nose and upper airways. Ultrafine particles travel much deeper. About 60% of particles in the 10 to 30 nanometer range deposit in the alveolar region, the deepest part of your lungs where oxygen enters your blood. Even at the larger end of the vape aerosol spectrum, around 100 nanometers, roughly 10% still reach the alveoli. Whatever chemicals those particles carry, whether nicotine, flavoring compounds, or metals, get delivered directly to tissue with an enormous surface area and thin barriers to the bloodstream.
The THC Vaping Risk: Vitamin E Acetate
Cannabis vape cartridges carry an additional hazard, particularly those purchased outside of regulated dispensaries. In 2019 and 2020, a wave of severe lung injuries called EVALI (e-cigarette or vaping product use-associated lung injury) swept the United States. Investigators found vitamin E acetate, a thickening agent used in illicit THC cartridges to cut costs and improve appearance, in the lung fluid of 94% of affected patients tested across 16 states. It was not found in healthy comparison subjects.
Among THC product samples linked to EVALI cases, 49% tested positive for vitamin E acetate and 24% contained other potential toxicants like medium-chain triglycerides. While regulated nicotine e-liquids are not typically associated with vitamin E acetate, any THC cartridge purchased from an unverified source carries this risk.
What Regulation Requires (and Doesn’t)
In the United States, the FDA requires manufacturers seeking to legally sell e-cigarettes to submit a premarket tobacco product application that includes measured levels of harmful and potentially harmful constituents in the aerosol, along with toxicological profiles covering cancer risk, respiratory toxicity, cardiac toxicity, and reproductive harm. This process is rigorous on paper, but it applies only to products that go through the formal approval pathway. Many products on the market, especially disposable vapes, have never completed this process. The result is that what you actually inhale from any given product may not have been independently tested or verified.