Propylene Glycol: Humectant, Solvent, and Penetration Helper

Propylene glycol is one of those ingredients that shows up in half your formulas doing three different jobs at once, and when you sit down to fill out a notification asking for its function, that versatility is exactly what trips people up.

The basics

INCI name: Propylene Glycol. CAS number: 57-55-6, a clean, single, well-defined identifier since it's a simple small molecule, not a botanical mixture. It's a small diol, meaning it has two hydroxyl groups, which is the structural feature behind most of what it does in a formula.

The three jobs it actually does

Humectant

Propylene glycol pulls moisture from the air toward the skin surface, which is the classic humectant function, similar in principle to glycerin though generally lighter in feel and less sticky. In lotions, creams, and hair products, this is usually its headline role.

Solvent

It's an effective solvent for ingredients that don't dissolve well in water alone, including some preservatives and actives. If you've ever wondered why a preservative blend calls for "dissolve in propylene glycol first," this is why. It's often the unsung workhorse ingredient that makes the rest of your formula actually come together in the mixing pot.

Penetration enhancer

At certain concentrations, propylene glycol can help other ingredients move through the outer skin layer more effectively. This is part of why it shows up in some topical formulations where an active ingredient needs help reaching deeper. In cosmetics, this function is usually secondary to its humectant or solvent role, but it's worth knowing it's doing this work quietly in the background.

Why "which function" matters for your filing

Most notification systems ask you to characterize what an ingredient is doing in your formula, at least in the sense of general categorization. Propylene glycol legitimately does more than one job depending on where and how much you use it. My practical advice:

1. Identify its dominant role in your specific formula. If you added a small amount mainly to help a preservative dissolve, its function there is solvent, not humectant, even though the same molecule can act as a humectant at higher concentrations elsewhere in the same product.
2. Don't overthink minor secondary roles. A notification form generally wants an honest, primary characterization of the ingredient's role, not an exhaustive list of every physical property it has.
3. Keep your formulation notes. If you ever need to explain your reasoning to a reviewer, having a clear record of why you classified the function the way you did saves a follow-up question.

Concentration matters more than the label suggests

Propylene glycol's behavior shifts meaningfully with concentration. A small percentage used as a carrier for a preservative behaves very differently in the finished product than a formula using it at a higher percentage specifically for its humectant properties. This isn't a restricted-substance issue, it's simply that the honest characterization of its function depends on how much you actually used and why, which is part of why "just copy what the last formula said" is a risky habit when you change your recipe even slightly.

Common mix-ups with similar-sounding ingredients

Propylene glycol gets confused with a couple of other glycols that have different profiles entirely:

Ingredient	INCI name	CAS number	Common role
Propylene glycol	Propylene Glycol	57-55-6	Humectant, solvent, penetration helper
Butylene glycol	Butylene Glycol	107-88-0	Humectant, solvent, often lighter feel
Ethylene glycol	Ethylene Glycol	107-21-1	Industrial use, not a cosmetic ingredient, do not confuse with the above
Glycerin	Glycerin	56-81-5	Classic humectant, plant or synthetic origin

That third row is worth staring at for a second. Ethylene glycol is not the same molecule as propylene glycol despite the similar name, and it is not a cosmetic ingredient. If a supplier document or a formula you copied from somewhere lists "ethylene glycol," that's worth double-checking immediately, since it's a genuinely different substance with a genuinely different safety profile, not a synonym or shorthand for propylene glycol.

What goes on the actual notification

For a Canadian filing, propylene glycol goes on the CNF under its INCI name with your concentration or concentration range, same as any other ingredient. It's not on the Cosmetic Ingredient Hotlist as a prohibited substance in ordinary cosmetic use, but it's always worth checking the current Hotlist directly for any use-specific restrictions before you file, since restricted-substance lists do get revisited and the safest habit is checking the live source rather than relying on last year's read of it.

If you're formulating multiple products that all use propylene glycol at different concentrations for different reasons, that's exactly the kind of ingredient-by-ingredient detail Cosmetic Comply is designed to track. It maps each ingredient to its correct INCI name and CAS number, carries the concentration through from your formula, and screens it against the current restricted list before your notification goes out.