Glycerin

A small (92 Da) trihydric alcohol that operates simultaneously as a humectant, a stratum corneum signalling molecule, and a corneocyte-maturation cofactor. It binds water via hydrogen bonding to its three hydroxyl groups, drawing moisture from both deeper skin layers and the atmosphere (above ~70% RH). Beyond passive humectancy it accelerates corneocyte maturation via aquaporin-3 signalling, supports desquamation enzyme function in low-humidity conditions, and integrates with stratum corneum lipid lamellae to reduce TEWL. One of the few cosmetic actives with positive in-use evidence at virtually every concentration from 0.1% to 50%.

2–10% (face); 10–30% (hand/body); higher in occluding masks

Effective at 2–10%; above 10% can feel sticky unless balanced with lighter humectants and silicone alternatives. Pairs with HA, PGA, sodium PCA, urea, and ceramides — the canonical NMF-mimicking stack.

In very low humidity, very high glycerin concentrations can theoretically pull water from the skin instead of into it — usually overstated, but layer over damp skin in dry climates.

The 'glycerin pulls water from the skin in dry air' concern is essentially overstated: occurs in narrow extreme-low-humidity conditions and is easily neutralised by application on damp skin or sealing with an occlusive. Source debate (palm-derived vs. coconut vs. rapeseed vs. soy vs. synthetic) is genuine on sustainability grounds but functionally irrelevant — the molecule is identical.

Underappreciated relative to its workload. Sold rarely as a hero, despite being the most reliable humectant on the shelf. The 2025 'glycerin maxxing' trend was a rare moment of consumer recognition of an ingredient that has done quiet heavy lifting for a century.