are theoretically possible, environmental samples usually contain a mixture of the linear isomer and 10 branched isomers. Telomerisation involves constructing the PFOS molecule using short chain (often 2-carbon) precursors and adding a sulfonate group as a final step. This production process results in 100% linear PFOS. This production method, whilst cleaner and resulting in a much more pure product than ECF, is not known to have been widely used except for the production of reagent grade PFOS and analytical standards. Indirect routes Perfluorooctylsulfonyl compounds degrade to PFOS. Examples include N-methyl perfluorooctane sulfonamidoethanol (N-MeFOSE), a carpet stain repellent, and N-ethyl perfluorooctane sulfonamidoethanol (N-EtFOSE), a paper treatment. Also perfluorooctanesulfonamide is a precursor. About 50 precursors were named in the 2004 proposed Canadian ban on PFOS. Degradation PFOS does virtually not degrade under environmental conditions and is thus highly persistent. Waste water treatment plants are also unable to degrade PFOS. On the other hand, precursors are transformed to PFOS in waste water treatment plants. Properties The C8F17 subunit of PFOS is hydrophobic and lipophobic, like other fluorocarbons, while the sulfonic acid/sulfonate group adds polarity. PFOS is an exceptionally stable compound in industrial applications and in the environment because of the effect of aggregate carbon–fluorine bonds. PFOS is a fluorosurfactant that lowers the surface tension of water more than that of hydrocarbon surfactants. Uses Perfluorooctanesulfonic acid is usually used as the sodium or potassium salts. PFOS was the key ingredient in Scotchgard, a fabric protector made by 3M, and numerous stain repellents. PFOS, together with PFOA, has also been used to make aqueous film forming foam (AFFF), a component of fire-fighting foams, and alcohol-type concentrate foams. PFOS compounds can also be found in some impregnation agents for textiles, paper, and leather; in wax, polishes, paints, varnishes, and cleaning products for general use; in metal surfaces, and carpets. In the semiconductor industry, PFOS is used in multiple photolithographic chemicals including: photoacid generators (PAGs) and anti-reflective coatings (ARCs). It has been phased out in the European Union semiconductor industry due to health concerns. PFOS is the key ingredient in Skydrol, a fire-resistant hydraulic fluid used in commercial aviation. The most important emission sources of PFOS are metal plating and fire-fighting foams. Because of concerns about PFOS, F-53B has been used as a replacement for mist suppression in metal plating. Levels in humans Because of its chemical nature, PFOS will remain in the body for several years.