When you vape, you inhale an aerosol made up of far more than just “water vapor.” The base liquid contains propylene glycol, vegetable glycerin, nicotine, water, and flavoring compounds. But the heating process transforms these ingredients, producing a mix of ultrafine particles, toxic chemicals, and trace metals that travel deep into your lungs with every puff.
The Base Liquids: Propylene Glycol and Vegetable Glycerin
Every e-liquid starts with two carrier liquids: propylene glycol (PG) and vegetable glycerin (VG). These are the substances that produce the visible cloud. Propylene glycol is thinner and carries flavor more effectively, while vegetable glycerin is thicker and creates denser vapor. Most e-liquids blend the two in varying ratios, along with water, nicotine, and flavoring agents.
Both PG and VG are considered safe to eat, which is why they show up in food products. But “safe to swallow” is not the same as “safe to inhale.” When these liquids contact a hot coil, they undergo thermal degradation, breaking apart into smaller, reactive compounds. This is where the real concern begins.
Toxic Carbonyls From Heating
The heating coil in a vape device doesn’t just warm the liquid. It breaks down propylene glycol and vegetable glycerin into a group of chemicals called carbonyls, several of which are known toxins or carcinogens. The main ones are formaldehyde, acetaldehyde, acrolein, and acetone.
Formaldehyde is classified as a known human carcinogen. Acetaldehyde is considered possibly carcinogenic and causes irritation to the eyes, skin, and respiratory tract. Acrolein is a potent lung irritant. The amounts produced vary enormously depending on the device, but they’re consistently detected across products. In some testing conditions, carbonyl levels from e-cigarettes reached or exceeded the levels found in traditional cigarette smoke, particularly during longer vaping sessions or when coils overheated.
Temperature plays a decisive role. In one analysis, formaldehyde levels jumped from 0.21 micrograms at 200°C to 60.2 micrograms at 400°C, a nearly 300-fold increase. Even at a moderate coil temperature of 300°C, formaldehyde exceeded the threshold for acute respiratory toxicity in humans. Higher wattage settings, chain vaping, and dry or degraded coils all push temperatures higher and toxicant production along with them.
Heavy Metals From the Device Itself
The hardware contributes its own contaminants. Heating coils, solder joints, battery contacts, and even mouthpieces leach metals into the aerosol you inhale. The most commonly detected metals include nickel, chromium, lead, copper, cadmium, and cobalt. These aren’t present in the liquid before it’s heated. They come from the device components themselves, dissolving into the aerosol as it passes over hot metal surfaces.
These metal particles are extremely small, often in the 20 to 300 nanometer range, which means they can penetrate deep into lung tissue. Chronic exposure to several of these metals is linked to respiratory damage, cardiovascular problems, and cancer risk. Nickel and chromium, for instance, are well-established lung carcinogens when inhaled.
Flavoring Chemicals
E-liquids come in thousands of flavors, and each one is created with chemical compounds that were originally approved for use in food. The problem, again, is that approval for ingestion doesn’t mean safety for inhalation. The lungs process substances very differently than the digestive system.
Diacetyl, the compound that gives a buttery flavor, is the most well-known example. Prolonged inhalation of diacetyl causes irreversible lung disease, sometimes called “popcorn lung,” first identified in factory workers who inhaled it occupationally. Related compounds like 2,3-pentanedione and acetoin carry similar risks. Cinnamaldehyde, used in cinnamon-flavored products, impairs respiratory immune cells and damages the protective lining of airways. Other flavoring agents like coumarin and maltol also disrupt the barrier function of airway tissue, potentially leaving the lungs more vulnerable to infection and irritation.
Volatile Organic Compounds
Beyond carbonyls, vape aerosol contains volatile organic compounds (VOCs) including benzene and toluene. Benzene is a known human carcinogen linked to leukemia. Toluene affects the nervous system. Testing has also detected ethanol, acetonitrile, and isopropyl alcohol in e-cigarette emissions. While concentrations of these VOCs are generally lower in vape aerosol than in cigarette smoke, they are not zero, and the long-term effects of chronic low-level inhalation through vaping remain an open question.
Nicotine Content and Delivery
Most e-liquids contain nicotine, and the form it takes matters. Older devices typically used freebase nicotine at relatively low concentrations. Newer pod-style devices often use nicotine salts, which are smoother to inhale at much higher concentrations. Products commonly range from 20 mg/mL up to 50 mg/mL or higher. At 40 mg/mL, nicotine salt formulations deliver nicotine to the bloodstream at rates comparable to a combustible cigarette. The European Union caps e-liquid nicotine at 20 mg/mL, but no similar federal limit exists in the United States, where concentrations well above that are widely sold.
Nicotine itself is not a carcinogen, but it is highly addictive, raises blood pressure, spikes adrenaline, and affects brain development in people under 25.
Ultrafine Particles That Reach Deep Into the Lungs
Vape aerosol isn’t gas. It’s a suspension of liquid droplets and solid particles, many of them extraordinarily small. Measurements show two main size clusters: nanoparticles around 11 to 25 nanometers and slightly larger submicron particles around 96 to 175 nanometers. For comparison, a human hair is about 70,000 nanometers wide. Particle concentrations reach 100 million to 1 billion particles per cubic centimeter of aerosol.
Particles this small don’t just sit in your mouth or throat. They deposit throughout the entire respiratory tract, including the deepest regions of the lungs where gas exchange happens. This is the same mechanism that makes air pollution so dangerous: the smaller the particle, the deeper it travels and the harder it is for your body to clear.
Vitamin E Acetate and Black Market Products
In 2019, a wave of severe lung injuries swept the United States, eventually identified as EVALI (e-cigarette or vaping product use-associated lung injury). The CDC identified vitamin E acetate as the primary cause. This oily substance was used as a cheap thickening agent in black market THC vape cartridges. When inhaled, it coats lung tissue and interferes with normal function. Vitamin E acetate was detected both in the products themselves and in fluid drawn from patients’ lungs across geographically diverse states. The outbreak largely subsided after the additive was identified and removed from circulation, but it illustrated how unregulated additives can turn vaping from a chronic risk into an acute emergency.