What Is in Vape Oil? Ingredients and Additives

Vape oil, usually called e-liquid or e-juice, is built on a surprisingly short list of intentional ingredients: a blend of propylene glycol and vegetable glycerin as the liquid base, nicotine in one of several chemical forms, and flavoring compounds. But what actually ends up in the aerosol you inhale goes well beyond that ingredient list. Solvents break down into toxic aldehydes when heated, flavoring chemicals react with the base liquid to form new compounds with unknown safety profiles, and metals leach from the heating coil into every puff. The gap between what is listed on a label and what reaches your lungs is wider than most users realize.

The Base Liquids

Propylene glycol (PG) and vegetable glycerin (VG) make up the bulk of any nicotine e-liquid, often accounting for more than 90% of the total weight. PG is a thin, nearly odorless synthetic liquid used as a food additive and pharmaceutical solvent. VG is thicker, slightly sweet, and derived from plant oils. Together they dissolve the nicotine and flavorings, and when heated they produce the visible cloud that mimics cigarette smoke. A study analyzing 50 commercial products found average PG and VG concentrations of roughly 538 and 482 milligrams per gram of liquid, respectively, with both compounds transferring efficiently into the aerosol at typical battery voltages.¹

Users often pick their PG/VG ratio based on personal preference. Higher PG blends deliver a stronger “throat hit” and carry flavor more sharply; higher VG blends produce thicker clouds and feel smoother. This ratio matters beyond aesthetics, though. When these solvents are heated by the coil, they don’t simply evaporate. Both PG and VG thermally degrade to produce formaldehyde, acetaldehyde, and acrolein, all of which are classified as toxic or carcinogenic.² Research has shown that this degradation begins at temperatures below 200°C, well within the operating range of many devices.³ The amount of aldehyde produced rises with higher power settings and longer puffs, which is why the question of “what’s in vape oil” can’t be fully separated from how the device heats it.

Nicotine Forms and Why They Matter

Nicotine in e-liquid comes in two main chemical forms: freebase nicotine and nicotine salts. Freebase nicotine is the older standard. It is more volatile and tends to produce a harsher sensation in the throat, especially at higher concentrations. That harshness effectively caps how much nicotine a freebase liquid can contain before it becomes unpleasant to vape, usually around 24 milligrams per milliliter or less.⁴

Nicotine salt formulations changed the game. By combining nicotine with an organic acid (benzoic acid is common), manufacturers lower the pH of the liquid, which converts most of the nicotine into its protonated salt form. The result is a smoother inhale even at concentrations of 50 mg/mL or higher. Products like JUUL popularized this chemistry, and it now dominates the disposable vape market. The lower pH counteracts the natural alkalinity of nicotine itself, keeping the liquid mild despite its high nicotine load.⁴

This chemistry also changes where nicotine deposits in the body. PET imaging of vapers found that nicotine salt aerosols deposit significantly more nicotine in the lower respiratory tract — the bronchi and deep lungs — with uptake increases of roughly 100% compared to freebase. Freebase nicotine, by contrast, deposited more in the mouth, throat, and trachea. Interestingly, both forms delivered about the same amount of nicotine to the brain, and users didn’t report different subjective effects.⁵ The long-term significance of deeper lung deposition isn’t fully understood yet, but it raises questions about whether salt-based products may carry different respiratory risks than freebase ones.

Synthetic Nicotine

A newer wrinkle is synthetic nicotine, manufactured in a lab rather than extracted from tobacco leaves. For a time, products using synthetic nicotine fell outside the FDA’s regulatory authority over tobacco products, creating a loophole that some manufacturers exploited. Chemically, the two are nearly identical. Tobacco-derived nicotine is almost entirely the (S)-enantiomer, and most commercial synthetic nicotine is enriched to match that same profile at 99% or higher purity. Analytical methods can distinguish the two only when the synthetic product retains an unusually high fraction of the (R)-enantiomer.⁶ Congress closed the regulatory loophole in 2022, but the episode illustrates how quickly the vaping industry adapts its chemistry to stay ahead of oversight.

Flavoring Compounds and Their Hidden Chemistry

Flavors are what draw many users to vaping in the first place, and their chemistry is arguably the least understood part of e-liquid. The flavoring industry developed these compounds for food, where they pass through the digestive system. Inhaling them is a completely different exposure route, and safety data for inhaled flavorings barely existed before the e-cigarette boom.

The most studied concern is diacetyl, a buttery-tasting compound famously linked to severe lung disease in popcorn factory workers. In those occupational settings, heavy inhalation exposure led to bronchiolitis obliterans, an irreversible scarring and narrowing of the small airways.⁷ Testing of 51 flavored e-cigarette products found diacetyl in about three-quarters of them. A related compound, 2,3-pentanedione, and the flavor acetoin were also widespread, with at least one of the three detected in 92% of products tested.⁸ A separate analysis found that roughly half of diacetyl-containing products exposed users to levels exceeding the strict occupational safety limits set by the National Institute for Occupational Safety and Health, though these levels were still far below those measured in cigarette smoke.⁹

Cinnamaldehyde, the compound that gives cinnamon its flavor, is another concern. Lab studies showed that cinnamon-flavored e-liquids suppressed the function of immune cells in the lungs in a dose-dependent way, impairing the ability of macrophages to engulf pathogens.¹⁰ This doesn’t prove that vaping a cinnamon-flavored product will make you sick, but it does suggest that some flavoring chemicals may weaken your lungs’ first line of defense against infection.

Synthetic Coolants

If you’ve ever vaped anything labeled “Ice” or “Cool Mint,” you’ve inhaled synthetic cooling agents. The two most common are WS-23 and WS-3, chemicals that trigger the same cold-sensing receptors as menthol but without much of menthol’s taste. Testing of popular disposable e-cigarettes found WS-23 in every product analyzed, at concentrations averaging about 21 mg/mL, with some reaching 40 mg/mL.¹¹ These coolants were found even in flavors not marketed as “Ice” varieties. Risk modeling showed that margins of exposure for WS-3 and WS-23 fell below the threshold considered safe for most daily use scenarios, meaning regular vapers may be exceeding safe exposure levels for these compounds.

Lab studies on WS-23 found that it can impair the actin cytoskeleton in bronchial tissue, disrupting processes that depend on the structural scaffolding of cells in the airway lining.¹² The unregulated addition of these coolants to e-liquids at high concentrations is one of the quieter concerns in vaping toxicology, partly because these agents are relatively new to the market and long-term data simply don’t exist yet.

Reactions You Didn’t Sign Up For

One of the less obvious issues with e-liquid is that its ingredients don’t just sit there waiting to be heated. Aldehyde-based flavoring compounds, including vanillin, benzaldehyde, cinnamaldehyde, and citral, chemically react with the PG and VG solvents while the liquid is still in the bottle. These reactions produce compounds called acetals, and research has found that upward of 40% of the original flavoring content can convert into these new acetal forms over time.¹³

This matters because the acetals that form are not the same chemicals that were tested and approved for food use. They have different toxicological properties. A substantial fraction of the flavor carbonyls in e-liquids were found to convert into acetals, significantly altering how the chemicals partition between liquid and gas phases.¹⁴ One study found that vanillin PG acetal was more toxic to airway epithelial cells than vanillin itself at high concentrations.¹⁵ The rate at which these reactions proceed depends on temperature, pH, and other additives present in the liquid.¹⁶ In practical terms, an e-liquid that has been sitting on a shelf for months may contain a substantially different cocktail of chemicals than a freshly mixed one, and neither the manufacturer nor the user would know the difference from the label.

Metals From the Hardware

The heating coil inside a vape device isn’t just a delivery mechanism — it’s an active source of contamination. Coils are typically made from nichrome (nickel-chromium alloy), kanthal (iron-chromium-aluminum), or stainless steel, and as they heat and cool repeatedly, metals leach into the surrounding liquid and aerosol. A study measuring metal concentrations across the e-cigarette system found that levels of lead in the aerosol were roughly 25 times higher than in the original e-liquid from the dispenser, with zinc about 30 times higher and nickel about 8 times higher.¹⁷

The problem appears to get worse as devices age. Research on popular disposable e-cigarettes found that heating coil elements like chromium and nickel increased up to 1,000-fold in concentration over the life of the device as the coil degraded during use.¹⁸ Higher-resistance coil settings also drove greater metal leaching, with lead, aluminum, and chromium concentrations rising significantly at higher resistance values.¹⁹

Beyond metals, early-generation cartridge-based devices were found to emit particles of tin, silver, iron, nickel, aluminum, and silicate, including nanoparticles smaller than 100 nanometers.²⁰ Some devices also shed glass particles from their internal wicking material, with fragments appearing in the aerosol after as few as 100 puffs.²¹ None of these materials appear on any ingredient label.

THC Vapes and the EVALI Crisis

Cannabis vape cartridges have their own distinct ingredient profile. An analysis of vape oil cartridges sold in California identified more than 100 terpenes and natural extracts, 19 cannabinoids, and potentially toxic additives including vitamin E acetate, polyethylene glycols, and medium chain triglycerides.²² In regulated markets, THC vapes typically use cannabis distillate thinned with terpenes or added to a carrier oil to improve viscosity and flow through the cartridge.

The unregulated market is where things went badly wrong. In 2019, a wave of severe lung injuries swept across the United States, eventually dubbed EVALI (e-cigarette or vaping product use–associated lung injury). Investigation revealed that vitamin E acetate, a cheap thickening agent added to illicit THC cartridges to dilute the oil while maintaining its visual appearance, was the likely culprit. Vitamin E acetate was found in the lung fluid of 94% of EVALI patients, and nearly all of those patients had either detectable THC in their samples or reported vaping THC products in the 90 days before falling ill.²³ The predominant demographic pattern among EVALI patients was healthy young men vaping illicit or informally sourced THC products.²⁴ The outbreak was largely contained once public warnings about black-market THC cartridges spread, but it remains the starkest example of how unregulated additives in vape oil can cause acute harm.

Contaminants Nobody Put There on Purpose

Beyond intentional ingredients, e-liquids can harbor biological and chemical contaminants that enter during manufacturing. Testing of 75 e-cigarette products sold in the United States found detectable levels of glucan, a microbial cell-wall component, in 81% of samples. Endotoxin, a bacterial component known to trigger airway inflammation, was found in about a quarter of them.²⁵ These aren’t ingredients — they’re contaminants that suggest unclean manufacturing conditions. Inhaled endotoxin exposure is well-established as a trigger for airway inflammation, even in people without pre-existing lung conditions.

Phthalate plasticizers have also been detected in e-cigarette aerosols, likely emitted from plastic components of the device during heating.²⁶ Phthalates are endocrine disruptors linked to reproductive and developmental harm, and while the concentrations in vape aerosol haven’t been studied as extensively as those in food packaging, their presence adds another layer of unintended exposure.

Label Accuracy Is Still a Problem

Even the ingredients that are listed on e-liquid labels aren’t always accurate. A market survey of disposable e-cigarettes found that nearly half the products tested deviated from their labeled nicotine concentration by more than 10%, with discrepancies ranging from 50% less nicotine than stated to about 14% more.²⁷ A broader review covering 545 e-liquid products across 23 studies found that nicotine strengths were most commonly mislabeled by 5 to 20%, with the majority testing lower than advertised. Some products labeled as “zero nicotine” contained substantial amounts of it, with one testing at nearly 24 mg/mL.²⁸

Regulation appears to help. In New Zealand, after vaping regulations were phased in between 2020 and 2023, the proportion of products within 10% of their labeled nicotine strength improved from 42% to 59%, and zero-nicotine products with detectable nicotine disappeared from later testing sets.²⁹ Discrepancies still remained, though, and in markets with less enforcement the situation is likely worse. For users trying to taper nicotine intake gradually, mislabeling can undermine the effort entirely.

How You Inhale Changes What You Get

The same e-liquid can deliver very different exposures depending on how you use the device. Vape devices generally support two inhalation styles: mouth-to-lung (MTL), where you draw vapor into your mouth and then inhale, and direct-to-lung (DTL), where you inhale deeply straight into your lungs. Modeling of aerosol behavior found that DTL vaping produced higher overall deposition in the lungs, with greater deposition in the deeper respiratory airways. MTL vaping concentrated more of the aerosol in the upper airways. Both styles showed relatively high total deposition, around 72 to 74% at the smallest and largest particle sizes.³⁰ Combined with the earlier finding that nicotine salt formulations shift deposition toward the lower lungs, a DTL device loaded with a high-strength nicotine salt liquid would push the highest doses of nicotine, metals, and flavoring chemicals deep into the most vulnerable tissue. That’s a combination the modern disposable vape market has made extremely common, particularly among younger users.