The substance exhaled when vaping is not steam or “water vapor,” but an aerosol—a suspension of ultrafine solid or liquid particles in a gas, similar to mist or fog. This aerosol is generated by heating an e-liquid to temperatures ranging from approximately 100°C to 250°C. This process causes the liquid to vaporize and subsequently cool into an inhalable mist. The exhaled aerosol is a complex mixture of ingredients from the original e-liquid, plus various chemical compounds formed during the heating and aerosolization process.
The Primary Matrix: PG and VG Aerosols
The visible, dense cloud comes overwhelmingly from the two main solvent carriers in e-liquids: Propylene Glycol (PG) and Vegetable Glycerin (VG). These compounds, known as humectants, produce the aerosol and act as a foundation to carry nicotine and flavorings. PG, a synthetic compound, tends to provide a stronger sensation in the throat, often referred to as a “throat hit,” and is a more efficient carrier for flavor compounds.
VG, derived from plant oils, is significantly thicker and is the primary agent responsible for the large, dense clouds associated with vaping. The ratio of PG to VG determines the physical characteristics of the aerosol. While both PG and VG are designated as “generally recognized as safe” (GRAS) for ingestion, this classification does not extend to their inhalation as a heated aerosol.
These chemicals were not previously used in aerosols intended for repeated, long-term inhalation, meaning the long-term effects of this exposure are not fully understood. When heated, the PG and VG base turns into ultrafine liquid droplets that penetrate deep into the lungs. These exhaled particles are the visual bulk of the cloud, carrying all other components of the e-liquid into the air.
Trace Contaminants and Degradation Products
Beyond the primary PG and VG base, the exhaled aerosol contains numerous trace substances, many of which are generated when the e-liquid is heated.
Nicotine and Flavorings
Nicotine, the primary addictive substance, is present in the exhaled cloud, though a significant portion is retained by the user. The amount of residual nicotine exhaled depends on the initial concentration in the liquid and the user’s puffing technique. Flavoring chemicals, often added in high concentrations, also contribute to the exhaled contaminants. Many flavorings, such as diacetyl (linked to a serious lung disease), are considered safe for consumption but not for inhalation, and these are aerosolized and exhaled.
Thermal Degradation Products
The thermal breakdown of PG and VG is a major source of harmful compounds. When the heating coil exceeds temperatures around 130°C, the solvents undergo thermal decomposition, producing volatile organic compounds (VOCs) like carbonyls and aldehydes. Specific degradation products frequently detected in exhaled air are known respiratory irritants and carcinogens:
- Formaldehyde
- Acetaldehyde
- Acrolein
Heavy Metals
The metal heating elements within the device can leach heavy metals into the e-liquid. These trace metals are then aerosolized and ultimately exhaled, posing an exposure risk to both the user and bystanders. Examples of these metals include:
- Nickel
- Chromium
- Lead
Retention Rates and Implications for Exhaled Air
What is exhaled is intrinsically linked to what is retained by the user’s body, a concept known as pulmonary retention. Studies show that the human respiratory system retains a large percentage of inhaled substances, particularly nicotine, PG, and VG. Nicotine retention rates are high, with typically over 90% of the inhaled dose absorbed systemically.
The lung tissues also retain a high percentage of the primary solvents, with retention rates for VG and PG often exceeding 85%. This high retention means that the user is absorbing the majority of the substance, including the harmful trace compounds. However, the remaining fraction, which is still substantial, constitutes the exhaled cloud and disperses into the immediate environment.
The exhaled aerosol, despite the high retention rate, still contains unabsorbed nicotine, unreacted PG and VG, degradation products like aldehydes, and heavy metal particles. This residual emission leads to a form of secondhand exposure for individuals nearby. Although the aerosol dissipates more rapidly than traditional cigarette smoke, the particles and chemicals can settle on surfaces, contributing to a “thirdhand” residue that can expose others to the contaminants over time.