Polyglutamic Acid
In plain terms
A large water-binding molecule made by fermentation (the sticky part of Japanese natto), used as a powerful humectant that sits on the surface and holds moisture. It is well tolerated; its safety file is lighter than the older humectants but reassuring so far.
What the evidence shows
Polyglutamic acid is a chain of glutamic acid, an amino acid, produced by fermentation; it is the substance that makes fermented soybeans, natto, stringy and sticky. As a humectant it is a heavyweight: it forms a water-holding mesh on the skin surface that binds many times its weight in moisture, and because the molecule is large it mostly stays on top rather than sinking in, which is part of why it is gentle. The tolerability evidence that exists is reassuring: it does not appear to irritate or sensitize, occluded patch tests on volunteers produced no redness, swelling, or itch, and laboratory testing showed no damage to genetic material. The honest caveat is breadth, not a red flag: polyglutamic acid has not been through a major regulator's cosmetic safety review the way glycerin or hyaluronic acid have, so the file is thinner. Nothing in it points to a problem, there is no hormone or cancer concern, and it is considered fine for daily use including in pregnancy. So it reads as no real concern, with confidence held to moderate because the safety record, while clean, is younger and lighter than the classics.
On the label
Listed as polyglutamic acid or PGA, usually among the humectants, often paired with hyaluronic acid. It sits on the surface and holds water, so like other humectants it works best under a moisturizer in dry air.