Approximately 400 brightener types are listed in the Colour Index, but fewer than 90 are produced commercially, and only a handful are commercially important. Generically, the C.I. FBA number can be assigned to a specific substance, however, some are duplicated, since manufacturers apply for the index number when they produce it. The global OBA production for paper, textiles, and detergents is dominated by just a few di- and tetra-sulfonated triazole-stilbenes and a di-sulfonated stilbene-biphenyl derivatives. The stilbene derivatives are subject to fading upon prolonged exposure to UV, due to the formation of optically inactive cis-stilbenes. They are also degraded by oxygen in air, like most dye colorants. All brighteners have extended conjugation and/or aromaticity, allowing for electron movement. Some non-stilbene brighteners are used in more permanent applications such as whitening synthetic fiber.