Most people vape a liquid mixture of propylene glycol, vegetable glycerin, nicotine, and chemical flavorings. That liquid, often called e-juice or vape juice, gets heated by a small battery-powered coil until it turns into an aerosol you inhale. But nicotine e-liquid is only part of the picture. People also vape THC oil, CBD, and a growing category of “wellness” products containing ingredients like melatonin, caffeine, and vitamins. Each of these carries a different set of ingredients and risks.
The Base Liquids in Every Vape
Regardless of what else is in the mix, nearly all e-liquids start with two base ingredients: propylene glycol (PG) and vegetable glycerin (VG). These are the solvents that carry the nicotine and flavoring and produce the visible cloud when heated.
Propylene glycol is thinner and carries flavor more effectively. It also creates a “throat hit,” that slight catch in the back of your throat that mimics the feel of cigarette smoke. Vegetable glycerin is thicker and sweeter, and it’s responsible for producing large, dense clouds. The ratio between the two determines a lot about the vaping experience. A 50/50 split is the most common starting point, offering a balance of flavor and cloud size. Ratios like 70/30 or 80/20 in favor of VG are popular with people who want bigger clouds but less flavor intensity. Mixes heavy on PG appeal to people trying to replicate the sensation of smoking a cigarette.
Both PG and VG are approved for use in food and cosmetics. That approval, however, covers eating or applying them to skin. Heating these compounds and pulling them into your lungs is a fundamentally different kind of exposure, and the long-term effects of doing so are still not fully understood.
Nicotine: Two Forms, Different Hits
Nicotine is the main reason most people vape. It comes in two forms in e-liquid: freebase nicotine and nicotine salts. The difference matters because it changes how quickly nicotine reaches your bloodstream and how the inhale feels.
Freebase nicotine has been the standard form in e-liquids for years. It’s typically sold in lower concentrations, such as 3 mg, 6 mg, or 12 mg per milliliter. It absorbs into the bloodstream more slowly and can feel harsh at higher strengths. Nicotine salts are made by combining nicotine with an organic acid (usually benzoic acid), which lowers the pH and makes higher concentrations smoother to inhale. Salt-based liquids commonly come in 10 mg or 20 mg strengths. They absorb faster into the body, delivering a nicotine rush that more closely mimics a traditional cigarette. This faster absorption is one reason pod-style devices loaded with nicotine salts became so popular so quickly, particularly among younger users.
Thousands of Flavors, Hidden Chemistry
Flavor is a huge part of what people vape. E-liquids come in virtually every flavor imaginable: fruits, desserts, candy, menthol, tobacco, drinks. These flavors are created using chemical compounds that are approved as safe to eat. Inhaling them is a different story.
Research from Duke University found that when common flavor chemicals for vanilla, cherry, citrus, and cinnamon mix with propylene glycol and glycerin at high temperatures, they form new compounds called acetals. In lab tests, these acetals were more effective at triggering molecular receptors involved in lung irritation than the original flavoring chemicals alone. These are the same receptors that sustain irritation and inflammation in people with asthma or smoke exposure. The concern is that repeated, low-level activation of these pathways could build into a chronic inflammatory response over time.
The flavoring chemicals are safe in a cookie or a bottle of lotion. The delicate tissue inside your airways offers far less protection against irritants than your gut or skin does.
Metals From the Heating Coil
The liquid is only part of what ends up in the aerosol. The heating element itself contributes contaminants. A study from Johns Hopkins found toxic metals in every aerosol sample analyzed across three types of devices: modifiable setups, pod systems, and disposables. The metals detected included nickel (a carcinogen), chromium, lead, manganese, cobalt, and arsenic. A portion of the samples exceeded regulatory inhalation safety limits for several of these metals.
Modifiable devices with customizable coils tended to produce higher overall metal concentrations. Pod systems and disposables had significantly higher levels of cobalt, which is toxic to lung tissue, and nickel. No device type was metal-free.
THC and Cannabis Oil
A significant number of people vape THC, the psychoactive compound in cannabis. THC vape cartridges contain concentrated cannabis oil, and in legal markets, these products typically list their ingredients and potency. The risks escalate with unregulated products.
In 2019, a nationwide outbreak of severe lung injuries, known as EVALI, hospitalized thousands and killed dozens. The CDC confirmed the primary culprit: vitamin E acetate, a cheap thickening agent added to illicit THC cartridges to dilute the more expensive cannabis oil. Vitamin E acetate is harmless when swallowed as a supplement or applied to skin. When vaporized and inhaled, it may interfere with surfactant, a natural fluid that keeps lung tissue flexible, or break down into toxic byproducts. The outbreak was overwhelmingly linked to black-market THC cartridges, not commercial nicotine e-liquids.
Since the outbreak, awareness of vitamin E acetate has pushed many illicit manufacturers to stop using it, but unregulated cartridges can still contain unknown diluents and contaminants.
“Wellness” Vapes: Melatonin, Caffeine, Vitamins
A newer category of vape products markets itself as nicotine-free and health-oriented. These devices deliver substances like melatonin, caffeine, vitamin B, vitamin C, CBD, essential oils, and even tea extracts. They’re popular among teens and young adults. A Stanford Medicine survey found that among users ages 25 to 40, caffeine and vitamin B were especially common choices.
The safety profile of these products is largely unknown. Just because a substance is safe to swallow does not mean it’s safe to inhale as a heated aerosol. Inhaling vaporized fat-like substances, including essential oils, glycerin, and certain flavor compounds, may interfere with normal lung function. For most of these substances, no one has established what dose, if any, is safe to breathe in.
These products also share base ingredients with nicotine vapes, including propylene glycol, glycerin, and flavoring chemicals already known to irritate airways. And because non-nicotine vapes fall outside FDA regulations covering nicotine e-cigarettes and ingested supplements, there are currently no restrictions on their ingredients, devices, or marketing claims.
What the FDA Has Actually Approved
Despite thousands of vape products on the market, very few have received formal FDA authorization. The agency requires e-cigarette manufacturers to submit applications demonstrating their products are “appropriate for the protection of public health.” As of early 2024, the products that received marketing orders are limited to tobacco-flavored options from a handful of brands: Vuse, NJOY, and Logic. No flavored products (fruit, mint, candy, or dessert) have been authorized.
The most popular brands among young users tell a different story. In the 2024 National Youth Tobacco Survey, 1.63 million middle and high school students reported currently using e-cigarettes. The most commonly reported brands were Elf Bar (36.1%), Breeze (19.9%), Mr. Fog (15.8%), Vuse (13.7%), and JUUL (12.6%). Most of these products, particularly the disposable flavored devices, have not received FDA authorization and are technically being sold illegally in the U.S. market.