What it is
Lactic acid is a small alpha hydroxy acid (AHA) that is highly water soluble and naturally occurring. In cosmetics it serves multiple roles: at low levels it functions as a pH adjuster and humectant, while at higher, properly formulated levels it can provide mild surface exfoliation. In sunscreens it most often appears in small amounts to fine-tune the formula's pH rather than as an active exfoliant.
Safety & warnings
Well studied and generally well tolerated, though as an AHA it can cause stinging or irritation on sensitive or compromised skin, especially at higher concentrations and low pH. AHA use can increase skin's sensitivity to the sun, so sun-protection guidance is relevant.
Origin & sourcing
Produced both synthetically and by fermentation of sugars; commercial cosmetic-grade lactic acid is commonly made through microbial fermentation.
Questions everyone asks
Why is lactic acid in a sunscreen?
In sunscreens it is usually present at low levels as a pH adjuster and humectant rather than as a strong exfoliant, helping keep the formula in a stable, skin-friendly range.
Is lactic acid an exfoliant?
It can be. As an alpha hydroxy acid it offers gentle surface exfoliation when formulated at suitable concentrations and pH, but small amounts mainly serve other roles.