E-liquid is the fluid used in electronic cigarettes and vape devices. It typically contains four ingredients: propylene glycol (PG), vegetable glycerin (VG), flavorings, and nicotine, though nicotine-free versions exist. When heated by a small coil inside the device, the liquid turns into an aerosol that the user inhales. You’ll also see it called vape juice, e-juice, or vape liquid.
The Four Main Ingredients
Propylene glycol and vegetable glycerin make up the bulk of any e-liquid, often 90% or more of the bottle. Both are thick, slightly sweet liquids classified as “generally recognized as safe” for oral consumption by the FDA, though their safety when inhaled as aerosol is a separate and less settled question. They serve as the carrier base that dissolves nicotine and flavorings and produces the visible vapor cloud.
PG is thinner and carries flavor more effectively. It also produces a sharper sensation in the throat, sometimes called “throat hit,” which mimics the feel of smoking a traditional cigarette. VG is thicker and sweeter, producing a smoother inhale and denser clouds of vapor. Lab studies have found that PG and VG alone, without added flavorings, do not appear to damage cells in culture. The potential risks come largely from what’s mixed into them.
Flavorings are the third ingredient. E-liquids come in thousands of flavor varieties, from tobacco and menthol to fruit, dessert, and candy profiles. These flavorings are typically food-grade compounds approved for eating, but “safe to eat” does not automatically mean “safe to inhale.” One well-known example: diacetyl, the chemical that gives a buttery taste, has been linked to serious lung disease when inhaled in occupational settings. A related compound, acetyl propionyl, raises similar concerns. Research has also shown that acetoin, sometimes used as a safer substitute, actually converts into diacetyl inside e-liquid over time, making it an unavoidable source of diacetyl exposure. Many reputable manufacturers have moved away from all three compounds, but they still appear in some products.
Nicotine is the fourth ingredient, though it’s optional. Labeled concentrations typically range from 0 to 36 mg/mL. Low-strength liquids sit around 3 to 6 mg/mL, medium around 12 mg/mL, and high-strength options reach 18 mg/mL or above. Pod-style devices often use concentrations of 20 mg/mL or higher.
Freebase Nicotine vs. Nicotine Salts
E-liquids use nicotine in one of two chemical forms. Freebase nicotine has a higher pH (around 8.9) and feels harsher on the throat at high concentrations. This is the traditional form used in most refillable vape devices, and it works best at lower nicotine strengths where the throat irritation stays manageable.
Nicotine salts combine nicotine with an acid to lower the pH to around 6.6, making it much smoother to inhale even at high concentrations. In salt-based e-liquids, roughly 98% of the nicotine is in a “protonated” form, meaning it carries an electrical charge. This protonated nicotine is less able to cross cell membranes quickly, which could slow how fast it enters the bloodstream compared to freebase nicotine, where only about 18% is protonated. Nicotine salts are the standard in compact pod systems like JUUL and similar devices.
How It Becomes Vapor
Inside every vape device is an atomizer: a small heating coil wrapped in or surrounded by a cotton wick. The wick draws e-liquid from the tank or pod through capillary action, keeping the coil saturated. When you activate the device, the battery sends power to the coil, heating the liquid to 200°C (about 390°F) or higher. At that temperature, the liquid rapidly vaporizes into a fine aerosol.
This heating process changes the chemistry of the liquid. Propylene glycol and vegetable glycerin can break down into carbonyl compounds, particularly aldehydes, when exposed to high heat. The risk increases with higher power settings and when the wick runs dry, creating what vapers call a “dry hit.” More advanced devices let users adjust voltage and coil resistance to control how much heat the liquid receives.
PG/VG Ratios and How They Matter
E-liquids are sold in various PG/VG ratios, and the blend affects the vaping experience. A 70/30 PG-to-VG ratio delivers a noticeably stronger throat hit, which some former smokers prefer. A 70/30 VG-to-PG ratio feels smoother and is popular with people who prioritize large vapor clouds, though in controlled testing, participants didn’t actually rate high-VG liquids as producing significantly bigger clouds than other ratios. The perceived difference may come from other factors like device power and airflow.
A 50/50 blend is a common middle ground, especially in smaller pod devices where a thinner liquid wicks more easily. Higher-VG liquids are thicker and work best in more powerful devices with larger wick openings. Using a very thick liquid in a device designed for thinner juice can lead to poor wicking, dry hits, and a burnt taste.
Flavoring Safety Concerns
The flavorings in e-liquid are the ingredient category that raises the most health questions. Cell studies have consistently found that the toxicity of e-liquids correlates with the number and concentration of flavoring chemicals present, not with nicotine content. Cinnamon and menthol flavors have drawn particular scrutiny. Cinnamaldehyde, the compound that gives cinnamon its taste, and menthol both activate specific receptor channels in lung cells that can trigger cell damage at high enough concentrations.
This doesn’t mean all flavored e-liquids are equally risky. Unflavored PG/VG bases show minimal cell toxicity on their own. The concern is specifically about certain flavoring compounds, and the dose matters. Products from manufacturers that test for harmful compounds and avoid ingredients like diacetyl, acetyl propionyl, and acetoin present a different risk profile than unregulated liquids with unknown contents.
Shelf Life and Storage
E-liquid has a shelf life of one to two years from the date of manufacture. The main enemy is oxidation: when the ingredients react with oxygen in the air, the liquid gradually changes color, shifting from clear or lightly tinted to yellow, then amber, then brown. Nicotine is especially prone to oxidation, which both darkens the liquid and reduces its potency over time.
Flavorings and sweeteners also degrade. You’ll notice the taste becoming muted, stale, or simply “off.” Storing e-liquid in a cool, dark place with the cap tightly sealed slows this process. Heat and direct sunlight accelerate it. Most quality manufacturers stamp a best-before date on the bottle, typically set at just under two years.
Manufacturing and Regulation
Professional e-liquid production ranges from basic mixing labs to pharmaceutical-grade cleanroom facilities. In controlled environments, manufacturers can monitor airborne contamination, humidity, temperature, and volatile organic compounds in real time, reducing batch-to-batch variation and preventing contamination from skin cells, bacteria, debris, or equipment residue. Stainless steel surfaces are standard for easy disinfection. Products labeled as USP-grade (United States Pharmacopeia) represent the highest manufacturing standard, with purity testing, potency analysis, and lot-to-lot tracking.
In the United States, e-liquids are regulated as tobacco products by the FDA. Any new product legally requires a Premarket Tobacco Product Application (PMTA). As of early 2024, very few e-cigarette products have received marketing authorization. NJOY’s tobacco-flavored disposable was among the first granted orders in 2022. The vast majority of flavored e-liquids on the U.S. market exist in a regulatory gray area, with many applications denied or still under review. In the UK and EU, nicotine-containing e-liquids are capped at 20 mg/mL and must meet specific labeling and safety standards.