Sodium Metabisulfite
In plain terms
A sulfite used as a preservative and antioxidant to keep products from browning or spoiling. It shows up in cosmetics, foods, and medicines, and it is the same family of preservative that some people react to in wine and dried fruit.
What the evidence shows
Sodium metabisulfite is a sulfite preservative, judged safe for cosmetic use by the Cosmetic Ingredient Review panel at the levels products use, and the reason it lands at depends is a specific group of people rather than a general hazard. It is a recognized contact allergen: in patch-test clinics it comes back positive in roughly four to six percent of people tested, enough that dermatologists describe it as a useful marker for cosmetic allergy, and it is a frequent, relevant cause of allergic contact dermatitis in settings like hairdressing. Sulfites are also a well-known trigger in sulfite-sensitive people, where they can set off asthma-like breathing symptoms or flushing, which is why foods have to declare them. Someone allergic to one sulfite tends to react to the others, so the whole group is best avoided once a reaction is known. For most people it does its preservative job without incident; for the sulfite-sensitive and the few percent who patch-test positive, it is one to recognize by name.
On the label
Listed as sodium metabisulfite, and by cousins like sodium sulfite, sodium bisulfite, or E223. If sulfites have ever given you a skin or breathing reaction, this is the family to watch for across cosmetics, foods, and medicines.