What It Is

Vegetable glycerin (1,2,3-propanetriol, C₃H₈O₇) is a three-carbon sugar alcohol derived from vegetable oils — typically palm, soy, or coconut — through hydrolysis of triglycerides. It is clear, viscous, odorless, and sweet-tasting. In e-liquid, VG is responsible for the thick, dense vapor clouds that distinguish vaping from smoking. It has a higher viscosity (about 1.4 times more viscous than PG) and a higher boiling point (~290°C vs. ~188°C for PG).

Safety Profile

A 2025 study by Findlay-Greene et al. in Toxicology Reports found that when free from contaminants or flavorings, e-liquids caused no significant impact on lung epithelial cells or pulmonary function in healthy or asthmatic human test subjects. This is the best-case scenario: pure PG and VG, no flavorings, no contaminants. Real-world e-liquids almost always contain flavorings, which changes the safety profile significantly.

Acrolein: VG's Most Dangerous Byproduct

When VG is heated above approximately 280°C, it decomposes to form acrolein — a cardiotoxic aldehyde classified by the World Health Organization as a hazardous air pollutant. Acrolein is the same toxic compound responsible for a significant share of cigarette smoke's cardiovascular damage. It causes acute lung injury, persistent airway inflammation, and arterial damage.

A 2021 study in Chemical Research in Toxicology used stable isotope tracing to confirm that thermal decomposition of VG in e-cigarette solvents leads to generation of both acrolein and glycidol, a potentially carcinogenic compound. The study's biomarker data showed that 23HPMA (3-hydroxypropyl-mercapturic acid) may serve as a relatively specific biomarker of e-cigarette use — meaning researchers can detect acrolein exposure from vaping in users' bodies.

The PG/VG Ratio Question

The ratio of PG to VG affects not just vapor production and throat hit, but also carbonyl formation. A 2024 study by Sun et al. found that commercially available PG/VG mixtures emit less carbonyl than pure forms, and that a 25:75 PG/VG ratio appeared to yield the lowest carbonyl release among tested ratios. This suggests that high-VG e-liquids may actually produce fewer harmful byproducts than 50/50 blends — though all ratios produce some byproducts.

For a deeper look at how PG/VG ratios affect your device choice, see our disposable vs. refillable cost comparison, which breaks down how different device types interact with different e-liquid viscosities.