Propylene Glycol

Overview

Propylene glycol (PG) is a small synthetic diol with three classic cosmetic roles: humectant (water-binding), solvent (for water-soluble and many oil-soluble actives), and penetration enhancer (helps actives cross the skin barrier). It is one of the most widely used cosmetic ingredients globally — found in everything from drugstore lotions to pharmaceutical topicals.

The molecule is much smaller than glycerin. That smaller size translates to:

• A lighter, less sticky skin feel than glycerin.
• More efficient solvent action for difficult-to-dissolve actives (salicylic acid, some retinoids, many fragrance compounds).
• Better penetration enhancement — PG transiently disrupts the stratum corneum lipid matrix, helping actives reach deeper layers.

PG has a long-standing safety profile when used as directed. There is a small (well-characterised) contact sensitisation rate, mostly in compromised skin. The “PG = antifreeze” trope confuses propylene glycol (cosmetic and food-safe) with ethylene glycol (toxic, automotive-only).

Propanediol (1,3-propanediol, often plant-derived) is the natural-positioned alternative — similar function, different molecule, much more expensive. Many formulators have switched to propanediol to avoid the negative consumer perception of PG.

Shelf life is 3+ years stored cool and dark.

What it does in a formula

The three roles overlap:

• Humectant — binds water from the air and from deeper skin layers, contributing to surface hydration.
• Solvent — dissolves water-soluble and many oil-soluble actives, useful for serums, tonics, and clear gels.
• Penetration enhancer — improves the delivery of active ingredients into the skin. This is especially relevant for actives like salicylic acid, niacinamide, and many botanical extracts.

In hair products, PG is also a useful conditioning agent and solvent for fragrances and hair colorants.

In preservation systems, PG at 5%+ can provide a mild boost to preservative efficacy by reducing water activity slightly — useful when paired with a borderline preservative system.

How to use

Add to the water phase. Compatible with virtually all cosmetic ingredients. Heat-stable.

Usage rates by product type:

• Face serums (humectant role): 2-10%
• Face creams (humectant + solvent): 1-5%
• Hair products (humectant + solvent): 2-10%
• Anhydrous tinctures and extract bases: 30-80%
• As a fragrance/extract solvent: 5-30% (depending on application)
• As a penetration enhancer: 1-5%

Best for / Worst for

Best for: mainstream face creams and lotions, hair products, fragrance and extract solvent applications, formulas with hard-to-dissolve actives, budget-conscious formulations.

Worst for: “natural-positioned” product lines (use propanediol or glycerin instead), customers with confirmed PG contact sensitivity (~3-5% of dermatitis patients show patch-test reactions to PG), products marketed as “PG-free” (obviously).

Common pitfalls

The “antifreeze” myth. Propylene glycol (food and cosmetic grade) is distinct from ethylene glycol (the toxic automotive antifreeze ingredient). The “PG = antifreeze” claim is a common confusion. PG is GRAS for food, used in pharmaceuticals, and has decades of established cosmetic safety data. Don’t reinforce the myth, but be honest that some customers do have legitimate sensitisation.

Over-using as solvent. PG above 20-30% can be drying and contributes to a tight skin feel. Pair with glycerin or other humectants to balance.

Stickiness with high glycerin. PG + glycerin together at high percentages (e.g. 10% + 10%) compound stickiness. Either reduce one or pair with a lighter humectant like sodium PCA.

Penetration enhancement of unwanted things. PG enhances penetration of whatever else is in the formula. This is useful for actives, but it also means contaminants, allergens, and irritants penetrate more deeply. Pay attention to the full formula when using PG as a delivery aid.

Confusing PG with propanediol. Propanediol (1,3-propanediol) is often plant-derived from corn fermentation, has a natural-positioned label, and behaves similarly but not identically. They are not the same ingredient and the substitution ratio is roughly 1:1 but with adjustment for feel.

Substitutes

• Propanediol (propanediol) — natural-positioned alternative, similar function, more expensive.
• Glycerin (glycerin) — heavier humectant, stickier feel, no penetration enhancement, much more skin-friendly perception.
• Butylene glycol (butylene-glycol) — similar small-molecule humectant + solvent, slightly different sensory.
• Pentylene glycol (pentylene-glycol) — fellow short-chain glycol with mild preservation boost.
• Sodium PCA (sodium-pca) — lightweight humectant, very different chemistry.