A common preservative used in many skincare products (washes, lotions etc.) and even in some baby products. Steer clear! - 

Quaternium-15 is a quaternary ammonium salt used as a preservative in many cosmetics and industrial substances. It acts as a formaldehyde releaser. 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.

It can be found under a variety of names, including: Dowicil 75; Dowicil 100; Dowco 184; Dowicide Q; 1-(3-Chloroallyl)-3,5,7-triaza-1-azoniaadamantane chloride; N-(3-chloroallyl) hexaminium chloride; hexamethylenetetramine chloroallyl chloride; 3,5,7-triaza-1-azoniaadamantane; 1-(3-chloroallyl)-chloride.

Quaternium-15 is an allergen, and can cause contact dermatitis in susceptible individuals.^[1] Many of those with an allergy to quaternium-15 are also allergic to formaldehyde. Allergic sensitivity to quaternium-15 can be detected using a patch test.^[2] It is the single most often found cause of allergic contact dermatitis of the hands (16.5% in 959 cases)^[3]