E-cigarette liquid, often called vape juice or e-liquid, is a mixture of four main components: a propylene glycol and vegetable glycerin base, nicotine (in most products), flavoring chemicals, and small amounts of water. That base alone makes up 80 to 97% of the liquid by volume. But what you inhale isn’t identical to what’s in the bottle. Heating transforms some of these ingredients into byproducts that weren’t part of the original formula.
The Base: Propylene Glycol and Vegetable Glycerin
Propylene glycol (PG) and vegetable glycerin (VG) are the two carrier liquids that make up the bulk of any e-liquid. Both are classified as humectants, meaning they absorb and hold moisture. PG is a thin, odorless synthetic liquid also used in food products, medications, and cosmetics. VG is thicker, slightly sweet, and derived from plant oils. Reputable manufacturers use USP (pharmaceutical) grade versions of both, which meet a purity standard above 99.8%.
The ratio of PG to VG changes how the liquid feels and performs. Higher VG concentrations produce larger, more visible clouds of vapor. Higher PG concentrations carry flavor more effectively. Interestingly, neither ingredient alone delivers much of a “throat hit.” Mixtures of the two, particularly around a 50/50 ratio, produce noticeably stronger sensations on the tongue and throat than either one by itself.
Common ratios you’ll see on labels include 50/50, 70/30 VG/PG (popular for sub-ohm devices), and max VG blends designed for cloud production. Some products use 100% VG for users who report sensitivity to propylene glycol.
Nicotine: Freebase vs. Salt
Most e-liquids contain nicotine, though zero-nicotine options exist. Nicotine comes in two forms. Freebase nicotine is the traditional form, typically sold in concentrations from 3 to 20 mg/mL. It has a higher pH, which means it gets harsher on the throat at higher concentrations. That’s why freebase liquids rarely exceed 18 or 20 mg/mL.
Nicotine salts pair nicotine with an organic acid (usually benzoic acid) to lower the pH. This makes higher concentrations smoother to inhale. Salt-based products commonly come in 20 to 50 mg/mL strengths, and at 40 mg/mL, nicotine salt formulations deliver nicotine to the bloodstream at rates comparable to traditional cigarettes. The European Union caps e-liquid nicotine at 20 mg/mL regardless of form, while U.S. regulations allow higher concentrations.
Flavoring Chemicals
Flavoring is where e-liquid composition gets complicated. Thousands of products are on the market, and each flavor profile relies on different chemical compounds. A strawberry e-liquid might contain benzyl alcohol, linalool, and benzaldehyde. A vanilla liquid could include vanillin, ethyl vanillin, ethyl maltol, and acetoin. Cinnamon flavor comes primarily from cinnamaldehyde. Banana flavor relies on isoamyl acetate. Tobacco flavors are among the most chemically complex, sometimes combining more than a dozen compounds including ethyl maltol, vanillin, and various lactones.
Many of these chemicals are classified as “generally recognized as safe” for eating, but that designation applies to ingestion, not inhalation. The lungs process substances very differently than the digestive system. Cinnamaldehyde is a good example: research published in the American Journal of Physiology found that cinnamon-flavored e-liquids suppressed the function of multiple types of immune cells in the lungs, including the cells responsible for engulfing bacteria. This effect was dose-dependent, meaning stronger concentrations caused more suppression. The chemical impaired immune cell function at concentrations well below the level that killed cells outright, so the damage isn’t something you’d necessarily feel immediately.
Diacetyl, the chemical responsible for buttery flavors, gained attention years ago for its link to a serious lung condition nicknamed “popcorn lung” in factory workers who inhaled it occupationally. While many manufacturers have removed diacetyl from their formulas, related compounds like acetoin still appear in some products.
What Heating Creates: Thermal Byproducts
The liquid in the bottle is only part of the picture. When propylene glycol and vegetable glycerin are heated by the device’s coil, they break down into smaller molecules. All tested e-cigarettes in laboratory studies produce three toxic aldehydes: formaldehyde, acetaldehyde, and acrolein. These are the same compounds found in cigarette smoke, though generally at lower levels.
At a moderate power setting of about 9 watts, 10 puffs from an e-cigarette produced roughly 8 to 40 micrograms of formaldehyde and 1.6 to 5.8 micrograms of acrolein. For comparison, a single conventional cigarette produces about 74 micrograms of formaldehyde and 120 micrograms of acrolein. So at normal use, e-cigarettes generate considerably less of these compounds than combustible cigarettes.
Power matters enormously, though. When researchers increased wattage from 11.7 to 16.6 watts on the same device, formaldehyde output jumped from about 130 micrograms to over 800 micrograms per 10 puffs. That’s more than 10 times the output of a cigarette. Acetaldehyde showed a similar pattern, rising from 23 micrograms to over 530. Running a device at high power or with a dry wick dramatically increases exposure to these harmful compounds.
Heavy Metals From the Device
E-liquid picks up contaminants from the hardware itself. Heating coils, solder joints, and other metallic components inside the device leach metals into the liquid and aerosol. Lead, mercury, cadmium, nickel, and chromium have all been detected in e-cigarette aerosols.
A large population study using data from Korea’s national health survey found that e-cigarette users had measurably higher blood levels of these metals. Cadmium levels were 61.4% higher in e-cigarette users compared to non-users, mercury levels were 13.7% higher, and lead levels were 10% higher. These metals accumulate in the body over time, and all three are established toxins linked to kidney damage, cardiovascular problems, and cancer risk at chronic exposure levels.
Vitamin E Acetate and Illicit Additives
In 2019, a wave of severe lung injuries swept across the United States, ultimately named EVALI (e-cigarette or vaping product use-associated lung injury). The CDC identified vitamin E acetate as the primary cause. This oily compound was being used as a thickening agent in black-market THC vape cartridges to dilute the product while maintaining its appearance.
Vitamin E acetate was found in lung fluid samples from 48 of 51 EVALI patients tested, and in none of the healthy comparison group. While vitamin E is harmless when swallowed as a supplement or applied to skin, inhaling it appears to coat the inside of the lungs and interfere with normal function. The outbreak was overwhelmingly tied to illicit THC products rather than commercially sold nicotine e-liquids, but it highlighted a real danger of unregulated or counterfeit vape products.
What Varies Between Products
Not all e-liquids are created equal. The specific chemicals you’re exposed to depend on the flavor profile, the nicotine type and concentration, the PG/VG ratio, and the device you use to vaporize it. A simple unflavored 50/50 liquid at low wattage exposes you to far fewer compounds than a heavily flavored, high-nicotine salt product run through a powerful sub-ohm device. Manufacturing quality also varies widely. Products from established manufacturers in regulated markets are more likely to use pharmaceutical-grade base ingredients and avoid known harmful additives, while unregulated or counterfeit products may contain undisclosed chemicals.