Perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) are two polyfluoroalkyl chemicals (PFAS)—a class of over 7,000 compounds with unique chemical structures that repel lipids and water. As a result, PFOA and PFOS have been used in numerous household products, such as nonstick cookware and stain-resistant carpets, and commercial applications such as fire-fighting foam. PFOS and PFOA are frequently referred to as "emerging contaminants," a label with no precise regulatory definition but generally understood to refer to chemicals for which there are few published standards designed to protect human health and the environment from perceived hazards. Many PFAS compounds are also often referred to as "forever chemicals" because of their persistence in the environment.