Citric Acid

Citric acid is the acid that makes a lemon taste sharp, though the citric acid in cosmetics is not squeezed from fruit. It is made by fermenting sugar with the mould Aspergillus niger, which is purer, cheaper and far less wasteful than pressing it out of citrus.

In almost every formula it appears in, its job is pH. The surface of skin is mildly acidic, and cleansers, toners and creams behave best and feel best when they are brought close to that. A small amount of citric acid solution is simply how a formulator moves a batch to its target pH. It works as a chelator too, binding the trace metal ions that would otherwise shorten a product's life, which is why it so often sits near the end of an ingredients list beside the preservative.

Citric acid is also an alpha hydroxy acid, and at high concentrations it behaves like one: in a 1997 study, a 20% citric acid lotion used for three months increased viable epidermal thickness and dermal glycosaminoglycan content in sun-damaged skin. That is a far higher level than the fraction of a percent needed to set pH, so where you see citric acid low down one of our ingredients lists, read it as a pH adjuster and not as an exfoliating acid.