Vaping delivers a complex aerosol containing far more than just nicotine and water vapor. When you inhale from an e-cigarette, you pull in a mixture of base liquids, nicotine, heavy metals, cancer-linked chemicals, flavoring compounds, and ultrafine particles small enough to reach the deepest parts of your lungs. Here’s a breakdown of each category.
Base Liquids: Propylene Glycol and Vegetable Glycerin
The foundation of any e-liquid is a blend of propylene glycol (PG) and vegetable glycerin (VG), which together make up roughly 80 to 97% of the liquid in a cartridge or tank. These two substances create the visible cloud when heated. They’re generally recognized as safe to eat, but their safety when heated and inhaled repeatedly is a different question, because heating them produces toxic byproducts covered below.
Nicotine
Most e-liquids contain nicotine in concentrations ranging from 3 to 36 mg/mL. Nicotine salt formulations, popular in pod-style devices, can reach 56 to 75 mg/mL, which is roughly four times the limit set by many countries. Nicotine is the addictive component, and at these higher concentrations it delivers a rapid hit comparable to or exceeding a traditional cigarette. Some e-liquids are sold as nicotine-free, but independent testing has occasionally found trace amounts even in those products.
Toxic Aldehydes From Heated Liquids
When propylene glycol and vegetable glycerin are heated on a metal coil, they break down into smaller, harmful molecules called aldehydes. The main ones are formaldehyde, acetaldehyde, and acrolein. Formaldehyde is a known human carcinogen. Acrolein is a potent irritant linked to cardiovascular and lung disease. These are the same toxic aldehydes found in cigarette smoke, though typically at lower levels in vaping aerosol under normal use.
The amount produced depends heavily on the device’s power setting. Increasing wattage from about 12 to 17 watts in one study caused dramatic spikes in aldehyde levels. A “dry puff,” where the coil overheats because it runs low on liquid, pushes aldehyde production even higher. If you’ve ever tasted a harsh, burnt hit from a vape, that’s essentially a concentrated dose of these breakdown products.
Heavy Metals From the Heating Coil
The metal components inside an e-cigarette, including the heating coil, vapor path, and electrical connectors, corrode over time and release metals directly into the aerosol. Testing of popular pod-style devices has detected nickel, chromium, copper, zinc, tin, and lead. Among the most concerning findings: lead reached up to 463 nanograms per 10 puffs, and nickel reached 373 nanograms per 10 puffs. Most heating coils are made from nichrome (a nickel-chromium alloy) or stainless steel, which explains why those two metals appear so consistently.
These aren’t metals you’d want in your lungs at any level. Nickel and chromium are classified as carcinogenic when inhaled, and lead exposure has no established safe threshold. The metals leach into the liquid while it sits in the pod and then transfer into the aerosol when heated.
Volatile Organic Compounds
E-cigarette aerosol contains several volatile organic compounds (VOCs), including benzene and toluene. Benzene is a well-established carcinogen that causes leukemia. While the concentrations detected in vaping aerosol are trace amounts, typically around 1 to 3 parts per billion, any inhalation of a known carcinogen adds risk, especially with daily use over years. Other VOCs detected include ethanol, acetonitrile, and isopropyl alcohol.
Flavoring Chemicals Linked to Lung Disease
Flavored e-liquids introduce their own set of inhaled chemicals. The most studied is diacetyl, a butter-flavoring compound that gained notoriety after workers in microwave popcorn factories developed bronchiolitis obliterans, a severe and irreversible scarring of the small airways commonly called “popcorn lung.” In testing of 51 flavored e-cigarettes, diacetyl was detected in 39 of them, spanning fruit, candy, and cocktail flavors, not just buttery ones.
Two related compounds, acetoin and 2,3-pentanedione (a diacetyl substitute), were also found in significant amounts. These flavoring chemicals are considered safe to swallow in food. Inhaling them is a fundamentally different exposure route, and the occupational health data on factory workers makes clear that the lungs do not tolerate them well.
Ultrafine Particles That Reach Deep Tissue
Every puff from an e-cigarette delivers a massive number of ultrafine particles. Measurements show two distinct size ranges: nanoparticles with a median diameter of 11 to 25 nanometers, and slightly larger submicron particles ranging from 96 to 175 nanometers. For perspective, a human hair is about 70,000 nanometers wide. Particle concentrations reach tens of millions to hundreds of millions per cubic centimeter of aerosol.
At the start of each puff, during the first fraction of a second, the user inhales nanoparticles only. These particles are small enough to deposit throughout the entire respiratory tract by diffusion. They can cross from lung cells into the bloodstream and lymph system, potentially reaching bone marrow, the spleen, the heart, and even the central nervous system by traveling along nerve fibers. Researchers believe these nanoparticles carry metals and other low-volatility chemicals deep into the body.
Tobacco-Specific Nitrosamines
Because nicotine in e-liquids is extracted from tobacco, trace amounts of tobacco-specific nitrosamines (TSNAs) can come along for the ride. These are potent carcinogens. Older, first-generation devices showed detectable levels, with one study finding up to 28 nanograms of a key nitrosamine per 150 puffs. Modern closed-system pods from major manufacturers have reduced these to below detection limits in most tests, representing a greater than 99% reduction compared to cigarette smoke. Still, their presence depends on the purity of the nicotine source, and cheaper or unregulated products may contain higher levels.
Vitamin E Acetate and Black-Market Additives
The 2019 outbreak of vaping-associated lung injury, known as EVALI, was strongly linked to vitamin E acetate, an oily additive used to dilute THC cartridges sold through informal channels. The FDA found vitamin E acetate in 50% of tested THC products at concentrations ranging from 23% to 88% of the liquid. It was detected in the lung fluid of EVALI patients but not in the lung fluid of healthy individuals. Other diluents found in black-market THC cartridges included medium chain triglycerides (in 29% of products) and polyethylene glycol. None of these substances belong in an inhalable product, and their presence is largely a problem of unregulated supply chains rather than commercial nicotine e-cigarettes.