Quaternium 15 is a preservative. It helps maintain product quality and performance over time. Some people raise concerns about preservatives, because a small number of individuals have allergies to them just as some people have allergies to nuts or bees. But preservatives play a very important role in food, cosmetics and household products. Without preservatives, many products would not last more than a week or two before being contaminated by bacteria, mold or yeast. So, we feel the best choice is to add preservatives in the smallest effective quantity to protect our products, but to list these preservatives in our formulas so that those with allergies in the family can make informed choices.