which take heated air from the engines and duct them to locations where ice can accumulate fluid systems which "weep" deicing fluid over wings and control surfaces via tiny holes Airport pavement Deicing operations for airport pavement (runways, taxiways, aprons, taxiway bridges) may involve several types of liquid and solid chemical products, including propylene glycol, ethylene glycol and other organic compounds. Chloride-based compounds (e.g. salt) are not used at airports, due to their corrosive effect on aircraft and other equipment. Urea mixtures have also been used for pavement deicing, due to their low cost. However, urea is a significant pollutant in waterways and wildlife, as it degrades to ammonia after application, and it has largely been phased out at U.S. airports. In 2012 the U.S. Environmental Protection Agency (EPA) prohibited use of urea-based deicers at most commercial airports. Deicing chemicals All chemical deicers share a common working mechanism: they chemically prevent water molecules from binding above a certain temperature that depends on the concentration. This temperature is below 0 °C, the freezing point of pure water (freezing point depression). Sometimes, there is an exothermic dissolution reaction that allows for an even stronger melting power. The following lists contains the most-commonly used deicing chemicals and their typical chemical formula. Salts Sodium chloride (NaCl or table salt; the most common deicing chemical) Magnesium chloride (, often added to salt to lower its working temperature) Calcium chloride (, often added to salt to lower its working temperature, attacks concrete Potassium chloride (KCl) Calcium magnesium acetate () Potassium acetate () Potassium formate () Sodium formate (HCOONa) Calcium formate () Organics Urea (), a common fertilizer Agricultural By-Products, generally used as additives to sodium chloride) Methanol (), scarcely used on roads Ethylene glycol (), scarcely used on roads Propylene glycol (), scarcely used on roads Glycerol (), scarcely used on roads Environmental impacts and mitigation Deicing salts such as sodium chloride or calcium chloride leach into natural waters, strongly affecting their salinity. Ethylene glycol and propylene glycol are known to exert high levels of biochemical oxygen demand (BOD) during degradation in surface waters. This process can adversely affect aquatic life by consuming oxygen needed by aquatic organisms for survival. Large quantities of dissolved oxygen (DO) in the water column are consumed when microbial populations decompose propylene glycol. Some airports recycle used deicing fluid, separating water and solid contaminants, enabling reuse of the fluid in other