An inorganic sulfite salt, used as a preservative and reducing agent and regulated in Annexes III and V.
What it is
Potassium metabisulfite is dipotassium disulphite (K2S2O5), an inorganic salt that releases sulfur dioxide and acts as a reducing agent. CosIng lists it as a hair waving or straightening, preservative and reducing ingredient. In the EU it is covered by entries in Annex III (substances subject to restrictions), for uses such as hair dye and hair straightening products, and Annex V (allowed preservatives).
Where it is used
Appears on the ingredient lists of a small number of products.
Limits of interpretation
The INCI list shows which ingredients are present, not how much of each. Above 1%, ingredients are listed in descending order of weight; below 1% the order is free.
See the EU regulation section for the applicable limits and conditions, which are expressed as free sulfur dioxide and differ between preservative and other uses.
Functions
- Hair waving or straightening
- Modifying the chemical structure of the hair, allowing it to be set in the style required (permanent waves or hair straightening).
- Preservative
- Exclusively or mainly intended to inhibit the development of micro-organisms in the cosmetic product. All authorised preservatives are substances in the positive list of Annex V to the Cosmetics Regulation 1223/2009.
- Reducing
- Changing the chemical nature of another ingredient by adding hydrogen (or removing oxygen).
EU regulation
- Maximum concentration
- 0.2% (as free SO2)
- As of
- 3 November 2026 · (EC) 2009/1223
- Product type
- (a) Oxidative hair dye products (b) Hair straightening products (c) Self-tanning products for the face (d) Other self-tanning products
- Maximum concentration
- (a) 0.67% (as free SO2) (b) 6.7% (as free SO2) (c) 0.45% (as free SO2) (d) 0.40% (as free SO2)
- As of
- 3 November 2026 · (EC) 2009/1223