the same four types. Deicing fluids containing thickeners (Types II, III, and IV) are also known as anti-icing fluids, because they are used primarily to prevent icing from re-occurring after an initial deicing with a Type I fluid. TKS fluid is similar to Type I fluid and is used by in-flight TKS ice protection systems. It can also be used for ground-based deicing. It conforms to different standards than Type I fluid, namely DTD 406B, AL-5, and NATO S-745. Chemical composition The main component of deicing fluid is a freezing point depressant (FPD), usually propylene glycol or ethylene glycol. Other ingredients vary depending on the manufacturer, but the exact composition of a particular brand of fluid is generally held as confidential proprietary information. Ethylene glycol (EG) fluids are still in use for aircraft deicing in some parts of the world because it has a lower operational use temperature (LOUT) than propylene glycol (PG). However, PG is more common because it is less toxic than ethylene glycol. In non-aviation contexts, deicing chemicals typically contain chloride salts, such as calcium chloride (CaCl2). These are prohibited in aircraft deicing fluids due to their corrosive properties. Based on chemical analysis, the U.S. Environmental Protection Agency has identified five main classes of additives widely used among manufacturers: Benzotriazole and methyl-substituted benzotriazole, used as corrosion inhibitor. Alkylphenol and alkylphenol ethoxylates, nonionic surfactants used to reduce surface tension. Triethanolamine, used as a pH buffer. High molecular weight, nonlinear polymers, used to increase viscoelasticity. Dyes used to aid in identification. TKS fluid (used by TKS in-flight deicing systems) contains 85% ethylene glycol, 5% isopropyl alcohol, and 10% water. Usage statistics The amount of fluid necessary to deice an aircraft depends on a wide variety of factors. Deicing a large commercial aircraft typically consumes between and of diluted fluid. The cost of fluid varies widely due to market conditions. The amount deicing service companies charge end users is generally in the range of US$8 to US$12 per diluted gallon (US$2.10 to US$3.20 per liter). The total annual usage of deicing fluids in the U.S. is estimated to be approximately , broken down as follows (figures from 2008, adjusted to show totals for undiluted fluid): Note that type II and type III fluids are rarely used in the United States. Measurement of performance Deicing fluid performance is primarily measured by Holdover Time (HOT), and Lowest Operational Use Temperature (LOUT). Holdover