waste is ammonia, whereas land-dwelling organisms convert the toxic ammonia to either urea or uric acid. Urea is found in the urine of mammals and amphibians, as well as some fish. Birds and saurian reptiles have a different form of nitrogen metabolism that requires less water, and leads to nitrogen excretion in the form of uric acid. Tadpoles excrete ammonia, but shift to urea production during metamorphosis. Despite the generalization above, the urea pathway has been documented not only in mammals and amphibians, but in many other organisms as well, including birds, invertebrates, insects, plants, yeast, fungi, and even microorganisms. Adverse effects Urea can be irritating to skin, eyes, and the respiratory tract. Repeated or prolonged contact with urea in fertilizer form on the skin may cause dermatitis. High concentrations in the blood can be damaging. Ingestion of low concentrations of urea, such as are found in typical human urine, are not dangerous with additional water ingestion within a reasonable time-frame. Many animals (e.g. camels, rodents or dogs) have a much more concentrated urine which may contain a higher urea amount than normal human urine. Urea can cause algal blooms to produce toxins, and its presence in the runoff from fertilized land may play a role in the increase of toxic blooms. The substance decomposes on heating above melting point, producing toxic gases, and reacts violently with strong oxidants, nitrites, inorganic chlorides, chlorites and perchlorates, causing fire and explosion. Analysis Urea is readily quantified by a number of different methods, such as the diacetyl monoxime colorimetric method, and the Berthelot reaction (after initial conversion of urea to ammonia via urease). These methods are amenable to high throughput instrumentation, such as automated flow injection analyzers and 96-well micro-plate spectrophotometers. Etymology Urea is New Latin, from French , from Ancient Greek (ouron, "urine"), itself from Proto-Indo-European *h₂worsom. History Urea was first discovered in urine in 1727 by the Dutch scientist Herman Boerhaave, although this discovery is often attributed to the French chemist Hilaire Rouelle as well as William Cruickshank. Boerhaave used the following steps to isolate urea: Boiled off water, resulting in a substance similar to fresh cream Used filter paper to squeeze out remaining liquid Waited a year for solid to form under an oily liquid Removed the oily liquid Dissolved the solid in water Used recrystallization to tease out the urea In 1828, the German chemist Friedrich Wöhler obtained urea