E-liquid is the fluid used in vapes and e-cigarettes that gets heated into an inhalable aerosol. It typically contains four ingredients: a base of propylene glycol and vegetable glycerin, nicotine (though not always), and flavorings. When the device’s coil heats the liquid to roughly 315–510°F, it transforms into the vapor you inhale. No tobacco is burned in the process, which is why vaping is often framed as an alternative to smoking, though it carries its own set of health considerations.
The Base Ingredients: PG and VG
The bulk of any e-liquid is a blend of two colorless, slightly sweet liquids: propylene glycol (PG) and vegetable glycerin (VG). Propylene glycol is a thin, odorless liquid also used in food products and medications. Vegetable glycerin is thicker and derived from plant oils. Together, they create the vapor you see and feel when you exhale.
The ratio of PG to VG changes the vaping experience in noticeable ways. Higher PG concentrations produce a stronger “throat hit,” that sharp sensation at the back of your throat that mimics the feeling of smoking a cigarette. In a study comparing common ratios, participants rated a 70/30 PG/VG blend as having a significantly stronger throat hit than both a 50/50 blend and a pure VG liquid. VG, on the other hand, produces thicker, denser clouds and a smoother inhale. Commercial e-liquids span a wide range, from 70/30 PG/VG all the way to 100% VG for people who want maximum cloud production with minimal throat sensation.
Nicotine Types and Strength
Most e-liquids contain nicotine, though nicotine-free options exist. Nicotine strength is listed in milligrams per milliliter (mg/mL), and products range from as low as 3 mg/mL up to 50 mg/mL or higher. The two main forms used in e-liquid are freebase nicotine and nicotine salts, and they behave quite differently in your body.
Freebase nicotine is the traditional form. It works fine at lower concentrations but becomes harsh and irritating to inhale at higher levels. Nicotine salts solve this problem by combining nicotine with an acid, which lowers the pH and makes high-concentration liquids smooth enough to inhale comfortably. This is why pod-style devices like JUUL can deliver 40 or 50 mg/mL without burning your throat.
The difference isn’t just comfort. At the same concentration (20 mg/mL), nicotine salt e-liquid delivers nearly twice the peak blood nicotine level compared to freebase. At 40 mg/mL, nicotine salt produces blood nicotine concentrations similar to smoking a traditional cigarette. This matters because higher nicotine delivery can satisfy cravings faster, but it also increases the potential for dependence.
Flavorings and Their Risks
Flavorings give e-liquids their enormous variety, from tobacco and menthol to dessert, fruit, and candy profiles. These flavoring chemicals are generally the same ones used in food manufacturing, and the FDA has classified many of them as “generally recognized as safe” to eat. The key word there is “eat.” Inhaling a chemical and digesting it are fundamentally different exposures, and safety data for ingestion doesn’t automatically apply to the lungs.
The most well-documented concern involves a butter-flavored chemical called diacetyl. Workplace exposure to diacetyl in food factories has been linked to a serious and irreversible lung condition called obliterative bronchiolitis, sometimes called “popcorn lung” because it was first identified in microwave popcorn factory workers. A substitute chemical, 2,3-pentanedione, has shown similar respiratory effects in animal studies. Many e-liquid manufacturers have voluntarily removed diacetyl from their products, but with over a thousand flavoring chemicals identified as potential hazards by the flavorings industry, the long-term inhalation risks of many common additives remain poorly understood.
How the Device Turns Liquid Into Vapor
Inside every vape is a small metal coil wrapped around a wick, usually made of cotton. The wick absorbs e-liquid from a tank or pod, and when you activate the device, the coil heats up. At temperatures between roughly 315°F and 510°F (157–266°C), the liquid vaporizes into a fine aerosol that you inhale through the mouthpiece.
Temperature matters more than most users realize. At the lower end of that range, the liquid converts to aerosol cleanly. As temperatures climb, the PG and VG begin to break down and produce unwanted byproducts called carbonyls, which include compounds like formaldehyde. This breakdown accelerates when the wick runs dry, producing the acrid “dry hit” that experienced vapers learn to avoid. Devices with temperature control settings help prevent this by capping how hot the coil gets.
How to Tell If E-Liquid Has Gone Bad
E-liquid doesn’t last forever. Unopened bottles are generally good for one to two years, while opened bottles should be used within about six months. Once air enters the bottle, oxygen and bacteria speed up degradation. Nicotine is especially vulnerable: it oxidizes on contact with air, turning the liquid darker over time.
Several signs indicate your e-liquid has expired:
- Color change: A shift from the original color toward amber or brown suggests nicotine oxidation.
- Separation or cloudiness: The ingredients should mix completely. Visible layers, floating particles, or a cloudy film mean the liquid has broken down.
- Flavor changes: Muted, bland, or flat taste means the flavorings have degraded. A harsh, peppery, or bitter taste points to oxidized nicotine.
- Weak vapor production: If hits feel noticeably thinner than usual with a fresh coil, the liquid itself may be past its prime.
Store e-liquid in a cool, dark place with the cap tightly sealed. Writing the open date on the bottle is a simple way to track freshness.
Safety Around Children and Pets
E-liquid containing nicotine is genuinely dangerous if swallowed, especially for small children. The FDA warns that accidental exposure can cause seizures, respiratory arrest, coma, and death. Even skin contact can cause burning and irritation. The liquid is often brightly colored and comes in sweet flavors, which makes it particularly attractive to young kids.
Store all e-liquid in its original child-resistant packaging, in a locked or elevated location out of reach and sight of children and pets. If a cap twists, tighten it completely after every use. Clean up spills immediately with soap and water. If anyone swallows e-liquid, call Poison Control at 1-800-222-1222 right away. If it contacts your skin, wash thoroughly with soap and water. If it gets in your eyes, rinse with water and get medical attention if your vision changes.