How Humectants Work
Humectants are hygroscopic molecules — they attract and bind water molecules through hydrogen bonding. In skincare, they draw water from two sources: the dermis (when atmospheric humidity is low) and the environment (when humidity is above ~70%). The practical implication: in dry environments, humectants alone can draw water out of your skin. They must always be sealed with an emollient or occlusive.
Hyaluronic Acid: The Molecular Weight Difference
Hyaluronic acid (HA) is a glycosaminoglycan naturally found in the dermis. Its ability to hold up to 1000× its weight in water makes it the benchmark humectant. However, not all HA is equal — molecular weight determines depth of effect:
| Molecular Weight | Size | Where It Works | Effect |
|---|---|---|---|
| High MW (1000–1800 kDa) | Large | Surface of stratum corneum | Surface hydration, plumping effect, barrier film |
| Medium MW (100–300 kDa) | Medium | Upper epidermis | Deeper hydration, some anti-inflammatory |
| Low MW (5–50 kDa) | Small | Deeper epidermis | Deeper penetration; can trigger mild inflammation at very low MW |
| Hydrolysed HA / Sodium HA | Varies | Epidermis | More water-soluble, better for layering |
The most effective HA serums contain multiple molecular weights to address all epidermal layers simultaneously.
Glycerin: The Underrated Workhorse
Glycerin (glycerol) is arguably the most effective topical humectant — and one of the cheapest. At concentrations of 20–40%, it outperforms hyaluronic acid in TEWL reduction studies. It also:
- Activates aquaporin-3 channels in keratinocytes — facilitating water transport through cell membranes
- Assists in skin barrier repair by supporting lipid organisation
- Has antimicrobial properties at higher concentrations
Panthenol (Pro-Vitamin B5)
Panthenol converts in skin to pantothenic acid (Vitamin B5), which is a cofactor in coenzyme A synthesis. Topically, it:
- Functions as a humectant (hygroscopic properties)
- Supports keratinocyte proliferation and wound healing
- Has anti-inflammatory properties — calms reactive skin
- Is well-tolerated by sensitive skin types and safe for compromised barriers
The essential rule: Humectant → Emollient → Occlusive. Apply in this order and you lock hydration at every layer. Skipping the occlusive step leaves humectants pulling water upward and evaporating it into dry air.
For the TEWL science behind humectant layering, see TEWL guide. For barrier lipids that seal humectants in, see Ceramides & Barrier Lipids.