Quaternium-15 is a quaternary ammonium salt. It is most commonly used as part of what is usually a large "cocktail" of preservatives in personal and skin care products that require a long shelf life. It is a known formaldehyde donor. . It can cause contact dermatitis, a symptom of an allergic reaction, especially in those with sensitive skin, on an infant's skin, or on sensitive areas such as the genitals.