Acetol (Hydryoxyacetone) is produced by degradation of various sugars.
Vegetal glycerin → Acetol → Propylene glycol: The acetol is formed by dehydration of a glycerol (vegetal glycerin -VG) molecule that undergoes intramolecular rearrangements, with a subsequent hydrogenation step, the acetol further reacts with hydrogen to form propylene glycol -PG) in the presence of a pre-reduced copper-chromite catalyst.
The second step of the reaction (the reaction of acetol to propylene glycol) is expected to be equilibrium limited, which means that acetol coexists with PG. Note that more propylene glycol is produced at lower reaction temperatures, typically around 180°C compared to 240°C.
Note also that in the presence of metallic catalyst and hydrogen, glycerol can be hydrogenated to propylene glycol or ethylene glycol whose toxicity is higher than propylene glycol.
In foods, acetol is formed by the Maillard reaction, a chemical reaction between amino acids and reducing sugars that gives browned food its desirable flavor, and can react further to form other compounds with various aromas.
It has first been described in the e-cigarette emissions by Sleiman et al. (2016) but has never been found to be harmful to humans.
M. Sleiman, J.M. Logue, V.N. Montesinos, M.L. Russell, M.I. Litter, L.A. Gundel, H. Destaillats 2016. Emissions from electronic cigarettes: Key parameters affecting the release of harmful chemicalsEnviron. Sci.Technol., 50, 9644-9651.