What It Is
Propylene glycol (1,2-propanediol, C₃H₈O₂) is a synthetic, colorless, odorless, hygroscopic liquid. It absorbs moisture from the air, which is why it works as a humectant in food, pharmaceuticals, and cosmetics. In e-liquid, PG serves three functions: it carries flavor compounds more effectively than VG, produces the "throat hit" that mimics combustible cigarettes, and thins the viscosity so the liquid can wick properly in devices with small coil openings.
The GRAS Misconception
PG was listed as Generally Recognized as Safe (GRAS) by the FDA in 1973 for use as a food additive. This is the single most misunderstood fact in vaping. The FDA's GRAS designation applies specifically and exclusively to ingestion — eating PG in food is safe. The National Academies of Sciences, Engineering, and Medicine (NASEM) report on e-cigarette toxicology states plainly: "GRAS substances are safe for ingestion, but not necessarily for other routes of administration like inhalation."
A 2024 review by Kassem et al. in Nicotine and Tobacco Research put it even more directly: "Food additives and GRAS substances are by the FD&C Act definition intended for use in food, thus safety is based on oral consumption; the term GRAS cannot serve as an indicator of the toxicity of e-cigarette ingredients when aerosolized and inhaled."
What Happens When PG Is Heated
When PG is heated inside a vaping device, it can undergo thermal decomposition. The critical threshold is approximately 350°C. Below that temperature, carbonyl formation is relatively low. Above it, production of formaldehyde and other toxic byproducts rises steeply. A method study found that high formaldehyde levels observed in some e-vapor products were likely due to heater temperatures exceeding 350°C.
The specific thermal decomposition products of PG include:
- Formaldehyde — IARC Group 1 human carcinogen. Not added during manufacturing; forms only when the base components are heated to decomposition temperatures.
- Acetaldehyde — IARC Group 2B possible human carcinogen. Detected across all nicotine formulations tested in a 2024 study published in Chemical Research in Toxicology.
- Methylglyoxal (MGO) — A toxic aldehyde that disrupts mitochondrial function, fatty acid metabolism, RNA binding, cytoskeletal organization, and DNA repair in human airway epithelial cells. A 2024 study in Toxicological Sciences showed that PG aerosols increased MGO levels in airway epithelia.
A 2025 UC Riverside study published in Frontiers in Toxicology exposed lab-grown human airway tissue to realistic concentrations of methylglyoxal and acetaldehyde. Methylglyoxal damaged mitochondria and weakened the actin cytoskeleton at surprisingly low concentrations. Acetaldehyde caused harm too, but required higher amounts.
Commercial PG Purity Concerns
Not all PG is the same grade. A 2024 study by Sun et al. found that commercially available PG used in e-liquids often contains impurities compared to analytically pure grades. GC-MS spectra provided evidence of these impurities, which may influence toxicity when aerosolized. This means that two e-liquids with identical PG percentages may have different purity profiles depending on supplier quality.
