Lactic Acid
In plain terms
An alpha hydroxy acid used to exfoliate and brighten, and at low strength a gentle humectant. Like other AHAs, it works by loosening the dead surface layer, which is also why it makes skin more sun-sensitive and why daily sunscreen comes with it.
What the evidence shows
Lactic acid is one of the two alpha hydroxy acids that safety reviews treat as the reference pair, alongside glycolic acid, and it is also a natural part of the skin's own moisture system. As an exfoliant it lifts dead surface cells to smooth and brighten; at low concentrations it mostly hydrates. Its one settled safety effect is the AHA effect: by thinning the outer layer, it leaves skin more reactive to the sun, which is why the FDA requires a sunburn alert on these products and why that sensitivity lasts about a week after you stop. The Cosmetic Ingredient Review cleared lactic and glycolic acid up to 10% at a pH of 3.5 or higher for home use, with stronger peels reserved for brief professional use. Europe's cosmetics committee took a more cautious line for leave-on consumer products, suggesting lower limits. More sun sensitivity is not the same as causing skin cancer, and when the FDA and the National Toxicology Program tested the related glycolic acid under UV for exactly that, they did not find a cancer link. The rest is dose and pH: low strengths properly formulated and paired with sunscreen are the tested zone, while high-percentage peels and skipped sun protection are where the trouble sits.
On the label
Listed as lactic acid or AHA, and as E270 on foods, where it is just an acidity regulator and not the concern. On skin, everyday strengths (up to about 10%, pH at or above 3.5) should carry a sunburn alert and pair with morning sunscreen; strong peels belong to professionals.