What it does
The EU's CosIng database lists two functions for sodium hyaluronate: humectant and skin conditioning.1 The US Cosmetic Ingredient Review describes hyaluronic acid and its sodium and potassium salts as skin conditioning agents, used in cosmetics at concentrations up to 2% at the time of its review.2
Hyaluronic acid is also made in the skin itself. A review of its role in skin ageing calls it the key molecule involved in skin moisture, with a unique capacity to retain water.3 Hyaluronic acid applied in a cosmetic is a separate matter from the hyaluronic acid your skin makes.
Sodium hyaluronate and hyaluronic acid
CosIng gives sodium hyaluronate's chemical name as "Hyaluronic acid, sodium salt".1 Hyaluronic Acid has its own entry, with antistatic, humectant, moisturising and skin conditioning among its functions.4 The Cosmetic Ingredient Review assessed the acid and its salts together.2
A front label that says "hyaluronic acid" may mean either ingredient; the list tells you which name was used. According to the same review, hyaluronic acid is obtained mainly from bacterial fermentation and from rooster combs.2 The name does not say which source a product used.
Molecular weight claims
Hyaluronic acid comes in different molecule sizes, and size can change how far it gets into skin. In a laboratory study that imaged sections of human skin with Raman spectroscopy, low molecular weight hyaluronic acid (20-300 kDa) passed through the stratum corneum, the skin's outermost layer, while high molecular weight hyaluronic acid (1,000-1,400 kDa) did not.5
One study on skin samples does not show what a particular product does on your face. And the ingredient name carries no molecular weight: "low molecular weight" or "multi-weight" on the front of a pack is the brand's description, not something the list can confirm. Some processed forms do have their own names: Hydrolyzed Sodium Hyaluronate, for example, is sodium hyaluronate broken down by hydrolysis.6
What sources say
The Cosmetic Ingredient Review panel found that hyaluronic acid was not a sensitiser in animal studies and that its widespread clinical use, mainly by injection, had been free of significant adverse reactions. It concluded that hyaluronic acid and its sodium and potassium salts are "safe for use in cosmetics as described in the safety assessment".2 That conclusion is about the ingredient as reviewed, not about any particular product.
How it appears on a label
As Sodium Hyaluronate. Related names, such as Hyaluronic Acid, Potassium Hyaluronate and Hydrolyzed Sodium Hyaluronate, are separate ingredients with their own entries.426 A serum may list more than one of them, and each name counts as its own ingredient.