Reducing

Ingredients with the reducing function in cosmetics work as reducing agents: they give up electrons and help to transfer some chemical bonds to a more "soft" state. In a practical sense, this is especially important for hair products, where such substances are able to temporarily break disulfide bonds in keratin. Due to this, the hair becomes more malleable to shape changes, for example, during curling or straightening. That is why thioglycolic acid and its salts are widely used in perming, straightening and depilation formulations; in PubChem bases for Thioglycolic Acid, Sodium thioglycolate and Ammonium Thioglycolate explicitly states the role of the reducing agent and the corresponding areas of use. (pubchem.ncbi.nlm.nih.gov)

On the skin, such components are usually not needed for daily care like classic moisturizers or emollients, but in special formulas they can participate in a controlled chemical effect on the keratin structures of the hair. This helps to loosen tight bonds in the hair and make it easier to remove hair or reshape it. At the same time, such substances require careful formulation and proper use, because at high activity they can irritate the skin and mucous membranes; for thioglycolic acid, PubChem also notes its use in depilators and hair products, and CIR indicates that safety depends on concentration and conditions of use. (pubchem.ncbi.nlm.nih.gov)

Typical examples of ingredients with this function are Thioglycolic Acid and Ammonium Thioglycolate; these substances also include some cysteine compounds, which are described in CIR estimates as reducing agents and agents for curling/straightening hair. For the consumer, this means that the reducing function is primarily related not to care comfort, but to changing the structure of the hair and the operation of active professional or special tools. (cir-safety.org)