Sodium PCA: The Humectant Your Skin Makes Itself

A molecule of 151 g/mol, no more. It makes up about 12% of the natural moisturizing factor that the stratum corneum produces to retain its moisture. Reintroducing it into a skincare product means giving the epidermis a raw material it already knows how to use.

• 151 g/mol. About 6,600 times lighter than a high molecular weight hyaluronate (≈ 1,000,000 g/mol): it diffuses into the stratum corneum instead of remaining as a film on its surface.
• ≈ 12% of endogenous moisture reserve. Just behind free amino acids (≈ 40%) and on par with lactate.
• 0.5 to 2.5% in formula. Beyond that, there's no benefit, and the texture becomes sticky.
• Never alone. Below 40% relative humidity, an active of this type must imperatively be sealed with a fatty substance — see the 60-second protocol.

In the outermost layers of the epidermis, a protein called filaggrin gradually breaks down into small soluble molecules. This cocktail has an English name that has become universal: the Natural Moisturizing Factor, or NMF. Its mission is simple and hard to imitate: to keep water inside the corneocytes so that the superficial layer remains flexible rather than brittle.

PCA, for pyrrolidone carboxylic acid, is formed by the conversion of glutamine from this degradation. It is therefore not an exotic import: it is an endogenous component, present in measurable quantities. The classically described distribution gives a useful order of magnitude — the percentages below being rounded averages, with the remainder grouping sugars, peptides, and minor organic acids.

This reserve is not immutable: dry cold, air conditioning, overly harsh cleansers, and the passage of years deplete it. A cosmetic supplement does not replace the physiological mechanism; it simply restores to the skin's surface a material it recognizes.

Where to look for it. Products sold in France, as throughout the European Union, carry a mandatory INCI list: the name appears as is, without translation. On an English-speaking bottle, the wording remains rigorously identical. At the usual dosage, it is almost always found in the second half of the list, after emulsifiers and before preservatives. Seeing it in third position would be suspicious rather than reassuring.

What matters more than its presence. A product that displays it without any fatty substance behind it will hardly keep its promise of comfort. Look at what follows in the list: a butter, an oil, an emollient ester. The benefits come from combinations, never from an isolated line.

The formulation constraint to be aware of. The independent Cosmetic Ingredient Review panel concludes that it is safe to use at the concentrations practiced, with a specific caveat: these salts should not be included in products that may contain nitrosating agents. Invisible to you, this rule explains why some laboratories prefer other options in complex systems.

Organic certified? Obtained by fermentation, it does not come from any identifiable crop: no organic label can therefore be individually attached to it, which does not prevent it from being included in formulas with a very high content of natural ingredients.

A word on forms. The pure crystalline form is mainly used by laboratories; in practice, almost all formulas start from the 50% liquid concentrate, which halves the concentration read on a technical data sheet.

Beyond the face. It is found in hair care: on dry hair, a conditioner containing it facilitates detangling and lightens the hair fiber. Same hygroscopic properties, different keratin — which is why some mists can be used interchangeably on the face and hair.