Vape juice, also called e-liquid or e-juice, is the fluid used in electronic cigarettes and vape devices. It typically contains four ingredients: propylene glycol (PG), vegetable glycerin (VG), nicotine, and flavorings. When heated by a small coil inside the device, the liquid turns into an aerosol that the user inhales. Despite looking like steam, that aerosol carries chemicals, ultrafine particles, and trace metals into the lungs.
The Four Core Ingredients
Propylene glycol is a thin, odorless, tasteless liquid classified as an alcohol. It carries flavor well and produces a noticeable throat hit, the slight burning sensation in the back of the throat that mimics smoking. Vegetable glycerin is thicker, slightly sweet, and responsible for the large visible clouds associated with vaping. Both are used widely in food, cosmetics, and pharmaceuticals, but their safety profile changes when they’re heated and inhaled repeatedly.
Nicotine is optional but present in most commercially sold vape juices. It comes in two forms. Freebase nicotine is the traditional type, typically sold in strengths from 3 to 12 milligrams per milliliter. Nicotine salts use an acid to lower the pH, making higher concentrations (20 to 50 mg/mL) smoother to inhale. This is why small pod devices can deliver nicotine levels comparable to a cigarette without the harsh throat sensation you’d get from the same concentration in freebase form.
Flavorings are food-grade chemicals adapted from the food industry. Thousands of flavor combinations exist, from fruit and dessert profiles to tobacco and menthol. Being safe to eat, however, does not make a chemical safe to inhale. The flavoring compound diacetyl, used for buttery and creamy tastes, has raised serious concern because inhaling it is linked to respiratory disease. Manufacturers have tried replacing diacetyl with alternatives like acetoin, but research published in Food and Chemical Toxicology found that acetoin chemically converts into diacetyl inside e-liquid bottles. The study’s authors recommended manufacturers stop using all three related compounds: diacetyl, acetyl propionyl, and acetoin.
How PG/VG Ratios Change the Experience
The ratio of propylene glycol to vegetable glycerin is the single biggest variable in how a vape juice feels. A 50/50 PG/VG blend is the standard starting point and the most common ratio in nicotine salt pods. It delivers moderate throat hit, solid flavor, and a modest amount of vapor. This ratio works best in smaller, lower-powered devices designed for mouth-to-lung inhaling, where you draw vapor into your mouth first and then inhale.
High-VG blends, usually 70% VG or higher, produce thick clouds and a much smoother inhale with little to no throat hit. These are designed for more powerful sub-ohm devices where users inhale directly into their lungs. If flavor intensity matters most, a 70/30 VG/PG ratio is a common choice. For maximum cloud production, some users go as high as 80/20. The tradeoff is that high-VG liquids are thicker and can clog smaller devices not built to handle them.
What Happens When It’s Heated
A metal coil inside the device heats the liquid through electrical resistance. The temperatures involved are significant. At 20 watts of power, the liquid reaches roughly 199°C (390°F), which is just above the boiling point of propylene glycol (188°C) but well below the boiling point of vegetable glycerin (290°C). At this low power, PG vaporizes preferentially while much of the VG stays behind. At 40 watts and above, the temperature climbs past 250°C, which is the boiling point of nicotine, meaning higher power settings deliver more nicotine per puff. Some devices can push temperatures past 315°C (600°F).
These temperatures matter because higher heat doesn’t just produce more vapor. It also generates more harmful byproducts. When PG and VG break down at high temperatures, they produce reactive aldehydes, a class of chemicals that damage cells. The aerosol also picks up metal particles from the heating coil itself, which is where the contamination problem begins.
Metals in the Aerosol
The liquid in the bottle doesn’t contain significant metals, but the aerosol that reaches your lungs does. As the coil heats, it releases tiny metal oxide particles into the vapor. A study in the Journal of Analytical Toxicology tested popular pod devices and found measurable levels of chromium, nickel, copper, zinc, tin, and lead in the aerosol. The concentrations varied dramatically by brand. One device produced up to 463 nanograms of lead per 10 puffs and 373 nanograms of nickel per 10 puffs, while others released amounts near or below detection limits.
These particles are primarily insoluble metal oxides, meaning they don’t dissolve easily in lung fluid and can accumulate in tissue. Cadmium, notably, was not detected above measurable levels in any of the devices tested. But the wide variation between brands and even between individual pods of the same product makes it difficult for consumers to know what they’re actually inhaling.
How Vape Juice Affects Lung Cells
Even without nicotine, vape aerosol triggers measurable changes in lung tissue. Exposure to PG/VG alone increases production of mucus proteins in airway cells. The aerosol also generates reactive oxygen species, unstable molecules that damage cell structures. In laboratory studies, small airway cells exposed to e-cigarette emissions produced eight times more of these damaging molecules than unexposed cells.
The damage goes deeper than irritation. Vape aerosol exposure causes both single-strand and double-strand DNA breaks in epithelial cells, the cells lining your airways. It disrupts mitochondria, the energy-producing structures inside cells, reducing their ability to power basic functions like clearing mucus through tiny hair-like structures called cilia. It also triggers inflammatory signaling: exposed cells release a cascade of proteins that recruit immune cells and sustain chronic inflammation. These effects occur regardless of whether nicotine is present in the liquid, pointing to the base ingredients and flavorings as independent sources of harm.
Shelf Life and Storage
Unopened vape juice stays usable for about one to two years. Once opened, quality drops noticeably within 6 to 12 months. Nicotine-containing liquids degrade faster than nicotine-free versions because nicotine oxidizes when exposed to oxygen, turning the liquid darker and producing a harsher, peppery taste.
Three factors accelerate breakdown: heat, light, and air. A bottle left in a hot car or on a sunny windowsill can degrade in weeks rather than months. Each time you open the cap, oxygen enters and begins reacting with the nicotine and flavorings. Store bottles upright in a cool, dark place with the cap tightly sealed. If the liquid has turned noticeably brown, smells off, or tastes harsh, it has oxidized past the point of normal use.
Regulatory Status in the U.S.
In the United States, vape juice is regulated as a tobacco product by the FDA. Any e-liquid sold legally must receive a premarket tobacco product authorization, a process that requires manufacturers to demonstrate their product is “appropriate for the protection of public health.” Relatively few products have received this authorization. The vast majority of flavored e-liquids on the U.S. market, particularly the disposable devices popular with younger users, are sold without FDA marketing orders and are technically illegal to sell. Enforcement, however, has not kept pace with the market, and unauthorized products remain widely available in stores and online.