Perfluorooctanesulfonic acid (PFOS) (conjugate base perfluorooctanesulfonate) is a chemical compound having an eight-carbon fluorocarbon chain and a sulfonic acid functional group and thus a perfluorosulfonic acid. It is an anthropogenic (man-made) fluorosurfactant, now regarded as a global pollutant. PFOS was the key ingredient in Scotchgard, a fabric protector made by 3M, and related stain repellents. The acronym "PFOS" refers to the parent sulfonic acid and to various salts of perfluorooctanesulfonate. These are all colorless or white, water soluble solids. Although of low acute toxicity, PFOS has attracted much attention for its pervasiveness and environmental impact. It was added to Annex B of the Stockholm Convention on Persistent Organic Pollutants in May 2009. History In 1949, 3M began producing PFOS-based compounds by electrochemical fluorination resulting in the synthetic precursor perfluorooctanesulfonyl fluoride. In 1968, organofluorine content was detected in the blood serum of consumers, and in 1976 it was suggested to be perfluorooctanoic acid (PFOA) or a related compound such as PFOS. In 1997, 3M detected PFOS in blood from global blood banks, although the company's internal documents indicate knowledge of this decades earlier, dating from the 1970s. In 1999, the U.S. Environmental Protection Agency began investigating perfluorinated compounds after receiving data on the global distribution and toxicity of PFOS, the key ingredient in Scotchgard. For these reasons, and USEPA pressure, the primary American producer of PFOS, 3M, announced, in May 2000, the phaseout of the production of PFOS, PFOA, and PFOS-related products. PFOS and PFOS-related chemicals are currently produced in China. Chemistry The main method used for the industrial scale production of PFOS is electrochemical fluorination (ECF). ECF is an electrolysis production method where a precursor of perfluorooctanesulfonyl fluoride is dispersed in a solution of hydrogen fluoride and electrolyzed. This production method, whilst economic and mainly results in PFOS, also results in shorter chain perfluoroalkyl substances being formed. PFOS predominates in the resultant mixture, however, if the reaction is allowed to continue this begins to favor the production of shorter chain PFAS. A distinct isomer ratio has been observed in PFOS produced by ECF, in the order of 70% linear PFOS, 25% branched and 5% terminal; this is not a function of the production process but rather that the precursor also exhibits this isomer ratio. ECF was the means by which 3M produced PFOS up until May 2000 when the company announced a phaseout of fluorosurfactants. 89 constitutional isomers of PFOS