What it is
Propylene glycol (propane-1,2-diol) is a viscous, colorless, water-miscible liquid belonging to the glycol family, containing two alcohol groups. In cosmetics it works mainly as a humectant that helps draw and hold water for hydration, and as a solvent that dissolves other ingredients into a formula. It also improves freeze-thaw stability so products tolerate temperature swings, and helps adjust viscosity. It can act as a penetration enhancer, but meaningful enhancement requires fairly high concentrations (around 10 to 25 percent) that are uncommon in finished cosmetics, where it is typically used at low levels.
Safety & warnings
Propylene glycol is a recognized contact allergen, with reported allergic contact dermatitis prevalence ranging roughly from 0.8 to 3.5 percent depending on concentration, which contributed to its designation as Allergen of the Year for 2018. In cosmetic references it is rated non-irritating at the low levels typically used, and high-concentration irritation concerns apply mainly to near-pure material. People with known propylene glycol sensitivity should avoid it, while most users tolerate the small amounts in cosmetics.
Origin & sourcing
It is a synthetic compound produced industrially mainly from propylene oxide, using either a non-catalytic high-temperature process or a catalytic process. An alternative route uses glycerol, a byproduct of biodiesel production. Global production was roughly 2.16 million tonnes per year as of 2018.
History
Propylene glycol is manufactured industrially on a large scale from propylene oxide and is a well-established cosmetic and industrial ingredient. It was named the American Contact Dermatitis Society's Allergen of the Year for 2018, reflecting both its very wide use and ongoing discussion of its sensitizing potential.
Questions everyone asks
What does propylene glycol do in skincare?
It acts mainly as a humectant that helps skin hold water, and as a solvent that dissolves other ingredients. It also helps products stay stable through temperature changes.
Is propylene glycol natural?
No, it is a synthetic ingredient made industrially, primarily from propylene oxide, though some is made from glycerol.
Can propylene glycol cause allergies?
It can be a contact allergen for some people and was named Allergen of the Year for 2018. Most users tolerate the low amounts used in cosmetics, but those with a known sensitivity should avoid it.