in diesel exhaust fluid (DEF), which is 32.5% urea and 67.5% de-ionized water. DEF is sprayed into the exhaust stream of diesel vehicles to break down dangerous emissions into harmless nitrogen and water. A component of animal feed, providing a relatively cheap source of nitrogen to promote growth A non-corroding alternative to rock salt for road de-icing. It is often the main ingredient of pet friendly salt substitutes although it is less effective than traditional rock salt or calcium chloride. A main ingredient in hair removers such as Nair and Veet A browning agent in factory-produced pretzels An ingredient in some skin cream, moisturizers, hair conditioners, and shampoos A cloud seeding agent, along with other salts A flame-proofing agent, commonly used in dry chemical fire extinguisher charges such as the urea-potassium bicarbonate mixture An ingredient in many tooth whitening products An ingredient in dish soap Along with diammonium phosphate, as a yeast nutrient, for fermentation of sugars into ethanol A nutrient used by plankton in ocean nourishment experiments for geoengineering purposes As an additive to extend the working temperature and open time of hide glue As a solubility-enhancing and moisture-retaining additive to dye baths for textile dyeing or printing As an optical parametric oscillator in nonlinear optics Physiology Amino acids from ingested food that are used for the synthesis of proteins and other biological substances — or produced from catabolism of muscle protein — are oxidized by the body as an alternative source of energy, yielding urea and carbon dioxide. The oxidation pathway starts with the removal of the amino group by a transaminase; the amino group is then fed into the urea cycle. The first step in the conversion of amino acids from protein into metabolic waste in the liver is removal of the alpha-amino nitrogen, which results in ammonia. Because ammonia is toxic, it is excreted immediately by fish, converted into uric acid by birds, and converted into urea by mammals. Ammonia () is a common byproduct of the metabolism of nitrogenous compounds. Ammonia is smaller, more volatile and more mobile than urea. If allowed to accumulate, ammonia would raise the pH in cells to toxic levels. Therefore, many organisms convert ammonia to urea, even though this synthesis has a net energy cost. Being practically neutral and highly soluble in water, urea is a safe vehicle for the body to transport and excrete excess nitrogen. Urea is synthesized