Sodium PCA
In plain terms
A strong humectant that is part of your skin's own natural moisturizing factor, used to draw and hold water. It is gentle and skin-identical; the one thing to watch is a formulation rule about what it should not be mixed with.
What the evidence shows
Sodium PCA is the salt of pyroglutamic acid, and it is one of the molecules your own skin makes as part of its natural moisturizing factor, the built-in humectant system that keeps the outer layer supple. Added to a product it is a powerful water-binder, holding many times its weight in moisture, and it is about as gentle as humectants come: the US cosmetic-ingredient panel found it non-irritating, non-sensitizing, and non-comedogenic up to high concentrations. The one caveat is a formulation rule rather than a fault in the molecule. The panel cautioned that sodium PCA should not sit in a product alongside nitrosating agents, because in that specific pairing it can react to form nitrosamines, some of which are carcinogenic. That risk is not sodium PCA acting on its own; it needs a second, nitrosating ingredient present, and those have become rare in modern cosmetics as the industry moved away from them. So on its own merits sodium PCA is a gentle, skin-identical hydrator with no real concern, and the nitrosamine pairing is the one thing to watch if you are reading a full ingredient list.
On the label
Listed as sodium PCA, usually mid-list among the humectants. On its own it is a gentle, skin-identical hydrator; the one thing to watch on a full ingredient list is that it should not be paired with nitrosating agents, a combination responsible formulators avoid.