Sodium Metabisulfite

Sodium metabisulfite is a sulfite antioxidant/preservative that can provoke irritation and clinically relevant hypersensitivity reactions (including asthma-type responses and contact dermatitis) in susceptible individuals, even at the low concentrations typically used in formulations. Patch testing and case reports support that while not universally irritating, it is a higher-risk ingredient for reactive/eczema-prone patients, so I score it as significant to reflect the need for caution and patch testing. Safety Notes: In consumer skincare, sodium metabisulfite is most often used as an antioxidant/reducing agent and preservative adjunct at very low levels (commonly ~0.01–0.1%) in both leave-on and rinse-off formulas, with some products listing it near the end of the INCI at trace-to-low percentages. Higher consumer-available levels are observed in certain high-strength cleansing/hair-removal/bleaching-type cosmetic preparations and some rinse-off products where sulfite functionality is leveraged more aggressively, approaching ~0.5–1.0% while still remaining OTC (these higher levels are much less common in leave-on due to odor/sulfite sensitivity and irritation risk). Regulatory frameworks do not set a single universal cosmetic maximum for this ingredient, so practical market use is primarily constrained by sensitization labeling considerations, product odor, stability, and local compliance for sulfite-releasing substances.