The ingredients listed above are what goes into an e-liquid. What comes out of the device is a different chemical mixture entirely. When e-liquid is heated, four processes occur simultaneously:
- Thermal decomposition of PG and VG — Both solvents break down into carbonyl compounds (aldehydes) when heated above their stability thresholds.
- Carbonyl formation — Formaldehyde, acetaldehyde, and acrolein are the most commonly detected carbonyls in e-cigarette aerosols.
- Flavoring degradation — Flavoring molecules degrade or react under heat, potentially forming additional harmful compounds.
- Metal particle release — Heating coils made of nickel, chromium, and iron can release metal particles into the aerosol.
Carbonyl Levels Measured in Studies
| Byproduct | IARC Classification | Measured Range in E-Cig Aerosol | Primary Source |
|---|---|---|---|
| Formaldehyde | Group 1 (carcinogenic) | 14-368 ng/mL | PG decomposition above 350°C |
| Acetaldehyde | Group 2B (possibly carcinogenic) | 2.1-4,676 ng/mL | Both PG and VG decomposition |
| Acrolein | WHO hazardous air pollutant | 0.3-10.1 ng/mL | VG decomposition above ~280°C |
| Methylglyoxal | Toxic aldehyde | 10-100x higher in used devices | PG decomposition; accumulates over device lifespan |
The Accumulation Problem: Why Used Devices Are More Toxic
A 2026 UC Riverside study published in ACS Omega found that several toxic aldehydes — including methylglyoxal, glyoxal, and formaldehyde — increased significantly after devices were used. MGO was reported to be 10 to 100 times more abundant in used devices compared with fresh e-liquid. This means that the last puffs from a disposable vape may contain far more toxic byproducts than the first puffs — a finding with direct consumer implications that is rarely communicated on packaging.
For users of disposable vapes, this is particularly relevant. Our best disposable vapes 2026 guide covers device-specific factors like coil material and power output that affect byproduct formation rates.
