Quaternium-15

Quaternium-15 is a formaldehyde-releasing preservative used at low concentrations, but it is a well-documented cause of allergic contact dermatitis on patch testing and can also sting/irritate compromised skin. In sensitive populations (eczema, barrier disruption), the sensitization risk and potential for significant flare-ups are high enough that I treat it as a very high irritancy ingredient from a patient-safety standpoint. Safety Notes: In legacy consumer cosmetics and personal care products (e.g., shampoos, liquid soaps, lotions, and some makeup) Quaternium-15 has been observed at low use levels around ~0.05% when used as part of a broader preservative system, with many formulations historically clustering near 0.1%. The highest over-the-counter use levels seen in commercial products are around 0.2%, which aligns with the typical maximum permitted concentration referenced in EU cosmetic regulation for this preservative; it was used in both rinse-off and leave-on products, though its market presence has declined substantially due to sensitization concerns.