Substance used in cosmetics to prevent bacterial and microbial contamination of products, particularly water-based formulas. Although there’s definitely a risk of irritation from these types of ingredients, the risk to skin and eyes from using a contaminated product is considered by many scientists to be even greater. 
- Lundov M, Moesby L, Zachariae C, Johansen J. Contamination versus preservation of cosmetics: a review on legislation; usage; infections; and contact allergy. Contact Dermatitis. 2009;60(2):70-8.