Vape Juice Ingredients: What You're Inhaling Beyond Nicotine
Propylene glycol is safe to eat. But is it safe to inhale thousands of times? Here's what's actually in e-liquid — and what happens when you heat it.
E-cigarette liquid contains a handful of ingredients. Propylene glycol. Vegetable glycerin. Nicotine (usually). Flavorings. That simplicity is part of the marketing. Four ingredients versus the 7,000-plus chemicals in combustible cigarette smoke. It sounds like an obvious win.
But the safety profile of an ingredient changes dramatically when you heat it to 200—300 degrees Celsius and inhale the resulting aerosol directly into your lungs. Not once. Not occasionally. Thousands of times per day, for years. Most of the safety data we have for these compounds was generated for oral ingestion or topical contact. The lungs are a different organ with different vulnerabilities, and the science of chronic inhalation exposure is still catching up to a product category that barely existed fifteen years ago.
This is not an article about whether vaping is evil. Public Health England’s widely cited estimate --- that e-cigarettes are roughly 95% less harmful than combustible cigarettes --- remains a reasonable working assumption for adult smokers switching entirely. But “less harmful than cigarettes” is a low bar, and “less harmful” is not “harmless.” Here is what is actually in the product, and what the research shows when you heat it and breathe it in.
The Base: PG and VG
Every e-liquid starts with a base of propylene glycol (PG), vegetable glycerin (VG), or a blend of both. Together, they typically make up 90% or more of the liquid by volume. PG carries flavor more effectively and produces a stronger throat hit. VG is thicker, sweeter, and produces the large visible vapor clouds that have become synonymous with vaping culture.
Propylene Glycol
PG holds FDA Generally Recognized as Safe (GRAS) status for use as a food additive. You will find it in salad dressings, ice cream, pharmaceutical preparations, and theatrical fog machines. It is one of the most common food-grade chemicals in commercial use.
But GRAS status specifically covers ingestion. The designation says nothing about safety when aerosolized and inhaled repeatedly over months or years. Long-term inhalation studies on PG are limited. Short-term occupational studies in theatrical workers exposed to PG-based fog show increased rates of respiratory symptoms --- coughing, throat irritation, chest tightness --- though these exposures are intermittent, not continuous.
The more concerning issue is thermal decomposition. When PG is heated at the temperatures common in e-cigarette coils, it can degrade to produce formaldehyde and acetaldehyde, both classified as carcinogens by the International Agency for Research on Cancer (IARC). A 2015 study in the New England Journal of Medicine detected formaldehyde-releasing agents in e-cigarette vapor at high voltage settings at levels 5 to 15 times higher than in combustible cigarette smoke. That study was criticized for using unrealistically high power settings that would produce a harsh, unpleasant “dry puff” --- conditions most users would avoid. Subsequent research at more realistic settings found lower but still measurable formaldehyde levels. The dose depends on the device, the wattage, and how often the coil is replaced.
Vegetable Glycerin
VG is also GRAS for food use. It is a thick, mildly sweet liquid derived from vegetable oils, common in baked goods, cosmetics, and pharmaceuticals. As a vape base, VG produces denser aerosol clouds and a smoother inhale.
The thermal decomposition concern with VG centers on acrolein. When glycerin is heated --- particularly at the higher wattages used in sub-ohm and mod-style devices --- it breaks down to produce acrolein, a potent respiratory irritant. IARC classifies acrolein as a Group 2A probable carcinogen. It is also one of the toxic compounds present in combustible cigarette smoke, though at higher concentrations. A 2017 study in Tobacco Control found that acrolein production increased significantly with coil temperature, with high-wattage devices producing levels approaching occupational exposure limits.
The practical takeaway: lower-wattage pod systems operating at lower temperatures produce meaningfully less thermal decomposition than high-wattage box mods. Device choice matters.
Flavorings: The Biggest Unknown
If PG and VG are the base, flavorings are the wildcard. There are thousands of flavoring compounds used across the e-liquid industry. Most are drawn from the food flavoring supply chain and hold GRAS status --- for ingestion. The phrase you will encounter repeatedly in this space is: safe to eat does not mean safe to inhale.
The respiratory system and the digestive system process chemicals differently. The gut has robust detoxification mechanisms, first-pass liver metabolism, and a mucosal barrier designed to handle foreign compounds. The lungs have none of that. They are designed to exchange gases, not to filter aerosolized flavoring chemicals. The surface area of the lungs is roughly 70 square meters --- an enormous, thin, permeable membrane with direct access to the bloodstream.
Diacetyl is the most infamous example. This butter-flavoring compound is responsible for the rich taste of movie theater popcorn. It is perfectly safe to eat. But factory workers exposed to diacetyl vapor developed bronchiolitis obliterans, a severe and irreversible scarring of the small airways colloquially known as “popcorn lung.” After this was linked to occupational inhalation exposure, many e-liquid manufacturers voluntarily removed diacetyl from their products. However, independent testing has continued to find it in some products, and the replacement compound --- 2,3-pentanedione --- has a similar toxicity profile in inhalation studies.
Other flavoring compounds raise concerns in cell and animal studies. Cinnamaldehyde, the compound that gives cinnamon its flavor, has shown cytotoxicity to human lung cells in vitro at concentrations relevant to e-cigarette use. Vanillin, one of the most common flavorings in sweet and dessert-flavored e-liquids, has been shown to form formaldehyde adducts when heated. Benzaldehyde, used in cherry and almond flavors, is a respiratory irritant at elevated concentrations.
The core problem is volume. There are thousands of flavoring compounds in use, each with its own thermal decomposition profile, and the vast majority have never been studied for inhalation safety. The flavor you choose may matter as much as the nicotine concentration or the device wattage.
The EVALI Crisis
In the summer of 2019, a cluster of severe lung injuries began appearing in emergency rooms across the United States. By February 2020, the CDC had documented 2,807 hospitalizations and 68 deaths from what was designated EVALI --- E-cigarette or Vaping product Use-Associated Lung Injury.
The cause was eventually identified: vitamin E acetate, a synthetic form of vitamin E used as a thickening agent in illicit THC (cannabis) cartridges. Vitamin E acetate is safe to swallow as a dietary supplement or apply to the skin. Inhaled into the lungs, it coats the alveolar surfaces and interferes with gas exchange, triggering severe lipoid pneumonia.
Critically, vitamin E acetate was not found in commercial nicotine e-liquids. The EVALI crisis was driven by black-market THC products, and it would be inaccurate to attribute those injuries to regulated nicotine vaping. But EVALI exposed something important: how unregulated the supply chain for vaping products was, and how dangerous novel inhalation ingredients can be when introduced without any safety testing. A compound that is harmless on the skin and in the stomach turned lethal in the lungs. The inhalation gap is not theoretical.
Metals From the Hardware
The ingredients list on an e-liquid bottle does not account for everything you inhale. The heating element itself contributes to the aerosol.
E-cigarette coils are typically made from kanthal (iron-chromium-aluminum alloy), nichrome (nickel-chromium), or stainless steel. The wick is usually cotton or silica. As the coil heats and degrades over time, metal particles leach into the aerosol.
A 2018 study by Olmedo et al. published in Environmental Health Perspectives analyzed aerosol from 56 e-cigarette devices and found significant levels of chromium, nickel, lead, and manganese. Median lead concentrations in the aerosol exceeded EPA drinking water limits by a factor of roughly 25. Nickel and chromium levels were similarly elevated. The concentrations were higher in aerosol than in the liquid itself, confirming that the metals were leaching from the coil and solder joints during heating.
Pod-style devices operating at lower temperatures generally produce less metal contamination than high-wattage mods with rebuildable atomizers. Coil age matters too --- older, degraded coils leach more metal. Replacing coils on schedule is not just about flavor; it is about reducing metal inhalation.
FDA PMTA: The Regulatory Bottleneck
In the United States, e-cigarettes are regulated as tobacco products by the FDA. Since 2020, manufacturers have been required to submit a Pre-Market Tobacco Product Application (PMTA) to keep their products on the market. The PMTA process requires manufacturers to demonstrate that the product is “appropriate for the protection of public health” --- a standard that considers both individual risk and population-level effects, including uptake by non-smokers and youth.
As of 2026, the FDA has authorized very few e-cigarette products through the PMTA pathway. NJOY received marketing orders for its menthol and tobacco-flavored products. Some Vuse products have been authorized. The vast majority of flavored e-liquids --- the thousands of candy, fruit, dessert, and menthol varieties that dominate the market --- remain without FDA authorization.
This does not mean they have been denied. Many applications are still pending review. Some have been denied and are being sold anyway under court orders or enforcement gaps. The FDA’s Center for Tobacco Products has been overwhelmed by the sheer volume of applications --- more than 26 million product submissions from over 500 companies. Enforcement has been inconsistent, and millions of unauthorized products remain available at retail and online.
The result is a market where most products have never undergone independent safety review of their specific ingredient formulations. The flavoring blend in your preferred e-liquid may never have been tested for inhalation toxicity by anyone --- not the manufacturer, not the FDA, not an independent lab.
The Inhalation Gap
The recurring theme across every ingredient category --- PG, VG, flavorings, even hardware --- is what might be called the inhalation gap. Compounds tested and approved for one route of exposure are assumed safe for a fundamentally different one. Eating propylene glycol is not the same as breathing it. Swallowing diacetyl is not the same as inhaling it. Applying vitamin E acetate to your skin is not the same as coating your alveoli with it.
GradeMyLabel’s vape ingredient category flags compounds with known thermal decomposition concerns, IARC classifications, and inhalation-specific risk data --- because the label on the bottle only tells you what went in, not what comes out when you heat it.