A typical vape contains a liquid mixture of two carrier solvents, nicotine, flavoring chemicals, and small amounts of unintended byproducts created when that liquid is heated. These carrier solvents alone make up 80 to 97% of most e-liquids, with nicotine and flavorings accounting for the rest. But the full picture of what you inhale goes beyond the ingredient label, because the heating process itself generates additional compounds that never existed in the original liquid.
The Two Base Liquids
Nearly every vape liquid starts with propylene glycol (PG) and vegetable glycerin (VG). Both are classified as humectants, meaning they absorb and hold moisture. They serve as the vehicle that carries nicotine and flavoring into an aerosol you can inhale. Propylene glycol is thinner and contributes more to “throat hit,” the sharp sensation at the back of your throat. Vegetable glycerin is thicker, slightly sweet, and responsible for producing larger, more visible clouds when you exhale.
Most commercial e-liquids blend these two in varying ratios. A 50/50 mix tends to produce the strongest airway sensations, while higher-VG blends (like 75% or 100% VG) maximize cloud production but feel smoother. Neither ingredient alone delivers the same throat sensation as a mixture of the two. These ratios matter because they change how the vape feels, how much vapor is visible, and how quickly the liquid wicks into the heating coil.
Nicotine: Freebase vs. Salt
Most vape liquids contain nicotine, though some are sold nicotine-free. The nicotine comes in two chemical forms: freebase and nicotine salt. Freebase nicotine is the older form, used in traditional e-liquids and refillable devices. It becomes harsh at higher concentrations, which limits how much nicotine a user can comfortably inhale per puff.
Nicotine salts changed the game for high-strength devices like pod systems. They’re made by combining freebase nicotine with an organic acid (commonly benzoic or citric acid), which lowers the pH of the liquid and changes how the nicotine behaves chemically. The result is a smoother inhale even at concentrations of 30 to 50 milligrams per milliliter, roughly two to three times what most freebase liquids deliver. This smoother delivery is why nicotine salts became the standard in compact, high-nicotine devices. The acids used to create these salts also interact with the base solvents when heated, adding another layer of chemical complexity to what’s actually produced in the aerosol.
Flavoring Chemicals
There are more than 7,700 commercially available flavored e-liquids, and the flavoring agents in them have largely not been evaluated for safety when inhaled. Many of these chemicals carry a “generally recognized as safe” (GRAS) designation from the FDA, but that label applies specifically to eating them, not breathing them into your lungs. The distinction matters.
Fruity flavors typically rely on aliphatic aldehydes, while sweet and spicy profiles use aromatic aldehydes like vanillin and cinnamaldehyde. Diacetyl, the butter-flavored compound linked to severe lung disease in popcorn factory workers, has been found in some e-liquids along with related compounds like acetoin and 2,3-pentanedione. Prolonged inhalation of these chemicals can cause irreversible lung damage.
Cinnamaldehyde, common in cinnamon and spice-flavored liquids, has received particular scrutiny. Laboratory research published in the American Journal of Physiology found that cinnamaldehyde suppresses the function of immune cells in the lungs in a dose-dependent way, meaning higher concentrations cause greater impairment. It specifically reduced the ability of macrophages (the immune cells that engulf and destroy pathogens) to do their job. Three different cinnamon-flavored e-liquids tested in the study all showed broad immunosuppressive effects.
What Heating Creates
The ingredient list on a vape liquid bottle doesn’t tell you everything you’ll inhale. When propylene glycol and vegetable glycerin are heated in the presence of oxygen, they begin breaking down into new compounds. This happens at temperatures as low as 133 to 175°C, well within the operating range of many vape coils.
The breakdown products include formaldehyde and acetaldehyde, both classified as toxic, along with acrolein, a potent lung irritant. The chemistry follows a cascade: glycerin first loses water to form intermediate compounds, which then fragment into these smaller, more reactive molecules. Formic acid and acrylic acid also form during this process and can damage kidney and lung tissue. These aren’t rare edge-case byproducts. They are a predictable result of heating these solvents.
The CDC has also confirmed the presence of volatile organic compounds like benzene and toluene in e-cigarette aerosols, alongside fine and ultrafine particles and carbonyls. These compounds are produced in small quantities compared to cigarette smoke, but they are present in the aerosol nonetheless.
Metals From the Coil
The heating element inside a vape is a small metal coil, typically made from nichrome (a nickel-chromium alloy) or Kanthal (iron-chromium-aluminum). As the coil heats and degrades over time, trace metals leach into the aerosol. The primary metals of concern are nickel, chromium, lead, manganese, and zinc. Nickel and chromium are classified as known carcinogens.
Research measuring biomarkers in vapers found a clear dose-response relationship: people exposed to higher concentrations of nickel in their aerosol had up to 72% more nickel in their urine and over 300% more in their saliva compared to those with the lowest aerosol exposure levels. Chromium showed a similar pattern, with the highest exposure group showing nearly 200% more chromium in saliva. These metals accumulate with regular use, and coil condition matters. Older, degraded coils release more metal into the vapor.
Ultrafine Particles
Vape aerosol is not water vapor. It consists of liquid droplets suspended in air, and those droplets come in two size ranges: nanoparticles measuring 11 to 25 nanometers across, and larger submicron particles ranging from 96 to 175 nanometers. Both are present in concentrations of tens of millions of particles per cubic centimeter of air.
Particle size determines how deep into your lungs the aerosol penetrates. Nanoparticles in the 11 to 25 nanometer range are small enough to reach the deepest regions of lung tissue, where gas exchange happens. Interestingly, “dry puff” tests run with an empty tank produced only nanoparticles, suggesting that the coil itself generates the smallest, most deeply penetrating fraction of the aerosol even without liquid present.
THC Vape Additives
THC vape cartridges, particularly those sold outside regulated dispensaries, can contain additional ingredients not found in nicotine vapes. The most notorious is vitamin E acetate, an oily chemical used to thicken or dilute THC oil. This additive was identified by the CDC as the primary cause of the 2019 outbreak of vaping-associated lung injuries (EVALI) that hospitalized thousands of people in the United States.
Vitamin E acetate is safe to swallow or apply to skin, but inhaling it is a different matter entirely. The oily substance coats the inside of the lungs and persists there, interfering with lung surfactant, the thin layer of molecules that keeps your airways open and functioning. This disruption triggers a form of lipoid pneumonia: inflammation caused by oil deposits in lung tissue. Every injured lung fluid sample tested during the EVALI investigation contained vitamin E acetate.
What Ends Up in the Room
Exhaled vape aerosol does release some nicotine into indoor air, but at very low concentrations. A study measuring air quality in the homes of vapers across four European countries found detectable airborne nicotine in about 72% of homes where someone vaped regularly. The levels, however, were extremely low, with a geometric mean of 0.01 micrograms per cubic meter. Fine particulate matter (PM2.5 and PM1.0) concentrations in vaping households were essentially the same as in non-vaping control homes. This suggests that while secondhand vape aerosol isn’t zero-exposure, the particulate burden it adds to a room dissipates quickly compared to cigarette smoke.