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What is Oxygen ? Oxygen is a chemical element with the chemical symbol O and atomic number 8. On Earth it is usually bonded to other elements covalently or ionically. Oxygen is one of the two major components of air. It is produced by plants , and is necessary for aerobic respiration in humans and animals. The word oxygen derives from two roots in Greek, (oxys) + (genes) . In the early 18th century, Antoine Lavoisier coined the name oxygen from the Greek roots mentioned above because he erroneously thought that it was a constituent of all acids..At standard temperature and pressure, oxygen exists as a diatomic molecule with the formula O2 ( Oxygen ), in which the two oxygen atoms are bonded to each other with the electron configuration of triplet oxygen. This bond has a bond order of two, and is thus often very grossly simplified in description as a double bond. Triplet oxygen is the ground state of the oxygen molecule. The electron configuration of the molecule has two unpaired electrons occupying two degenerate molecular orbitals. These orbitals are classified as antibonding, so the diatomic oxygen bond is weaker than the diatomic nitrogen bond, where all bonding molecular orbital's are filled. Though unpaired electrons are commonly associated with high reactivity in chemical compounds, triplet oxygen is relatively unreactive by comparison with most radicals.Singlet oxygen, a name given to several higher energy molecular oxygen in which all the electron spins are paired, is much more reactive towards common organic molecules. In nature, singlet oxygen is commonly formed from water , using the energy of sunlight. It is also produced by the immune system as a source of active oxygen. Carotenoids in organisms and possibly also in human and animals, play a major role in absorbing energy from singlet oxygen and converting it to the unexcited ground state. Liquid O2 ( Oxygen ) Liquid O2 ( Oxygen ) and solid O2 ( Oxygen ) are clear substances with a light sky-blue color. In normal triplet form they are paramagnetic due to the spin magnetic moments of the unpaired electrons in the molecule, and the negative exchange energy between neighboring O2 ( Oxygen ) molecules. Liquid oxygen is attracted to a magnet to a sufficient extent that a bridge of liquid oxygen may be supported against its own weight between the poles of a powerful magnet, in laboratory demonstrations. Liquid O2 ( Oxygen ) is usually obtained by the fractional distillation of liquid air. Oxygen is slightly soluble in water, but naturally occurring dissolved amounts are enough to support human and animal life .The common elemental oxygen on Earth, O2 ( Oxygen ), is known as dioxygen. Ozone, the less common triatomic allotrope of oxygen, is a poisonous gas with a distinct, sharp odor. It is thermodynamically unstable toward the more common dioxygen form. It is formed continuously in the upper atmosphere of the Earth by short-wave UV radiation, and also functions as a shield against UV radiation reaching the ground. Ozone has recently been found to be produced by the immune system as an antimicrobial Liquid and solid O3 (ozone) have a deeper blue color than ordinary oxygen, and they are unstable and explosive.A recently discovered allotrope of oxygen, tetraoxygen (O4), is a deep red solid that is created by pressurizing O2 ( Oxygen ) to the order of 20 GPa. Its properties are being studied for use in rocket fuels and similar applications, as it is a much more powerful oxidizer than either O2 ( Oxygen ) or O3 Molecular O2 (Oxygen ) Molecular oxygen (also called unbound oxygen , or dioxygen, O2 ( Oxygen ), a diatomic molecule) first appeared in significant quantities in Earth's atmosphere during God's creation ( Genesis 1:1 ). The presence of large amounts of free oxygen in the atmosphere , the atmospheric abundance of free oxygen and its gradual increase up to the present .Uptake of oxygen from the air is the essential purpose of respiration, so oxygen supplementation has found use in medicine (as oxygen therapy). People who climb mountains or fly in non-pressurized aero planes sometimes have supplemental oxygen supplies; the reason is that increasing the proportion of oxygen in the breathing gas at low pressure acts to augment the inspired oxygen partial pressure nearer to that found at sea-level. A notable application of oxygen as a very low-pressure breathing gas, is in modern spacesuits, where use of nearly pure oxygen at a total ambient pressure of about one third normal, results in normal blood partial pressures of oxygen. This trade-off of breathing gas content and needed pressure is important for space applications, because the issue of flexible spacesuits working at Earth sea-level pressures remains a technological challenge of aerospace technology.Oxygen is used in welding (such as the oxyacetylene torch), and in the industrial production of steel and methanol. Also, liquid oxygen finds use as a classic oxidizer in rocket propulsion. Oxygen presents two spectrophotometric absorption bands peaking at the wavelengths 687 and 760 nanometers. Some scientists have proposed to use the measurement of the radiance coming from vegetation canopies in those oxygen bands to characterize plant health status from a satellite platform. This is because in those bands, it is possible to discriminate the vegetation's reflectance from the vegetation's fluorescence, which is much weaker. The measurement presents several technical difficulties due to the low signal to noise ratio and due to the vegetation's architecture, but it has been proposed as a possibility to monitor the carbon cycle from satellites on a global scale.Oxygen, as a supposed mild euphoric, has a history of recreational use . However, the reality of a pharmacological effect is doubtful being a metabolic boost the most plausible explanation. Controlled tests of high oxygen mixtures in diving and other activities, even at higher than normal pressures, demonstrated no particular effects on humans other than promotion of an increased tolerance to aerobic exercise.In the 19th century, oxygen was often mixed with nitrous oxide to temper its analgesic effect. A stable 50% gaseous mixture (Entonox) is commonly used in medicine today as an analgesic. However, the common basic anaesthetic mixture is 30% oxygen with 70% nitrous oxide; the pain-suppressing effects, obviously, are due to the nitrous oxide and not to oxygen. Oxygen was first described by a Polish alchemist and philosopher in the late 16th century. Oxygen was more quantitatively discovered by the Swedish pharmacist some time before 1773, but the discovery was not published until after the independent discovery on August 1, 1774, who called the gas dephlogisticated air. As noted, the name reflects the then-common incorrect belief that all acids contain oxygen.Oxygen is the third most abundant chemical element in the universe by mass, after hydrogen and helium . Some of this oxygen was produced during the CNO cycle. Oxygen is the most common component of the Earth's crust (49% by mass), the second most common component of the Earth as a whole (28% by mass), the most common component of the world's oceans (86% by mass), and the second most common component of the Earth's atmosphere (20.947% by volume), second to nitrogen.Elemental oxygen occurs not only in the atmosphere, but also as solution in the world's water bodies. At 25° C under 1 atm of air, a litre of water will dissolve about 6.04 cc (8.63 mg, 0.270 mmol) of oxygen, whereas sea water will dissolve about 4.9 cc (7.0 mg, 0.22 mmol). At 0° C the solubilities increase to 10.29 cc (14.7 mg, 0.460 mmol) for water and 8.0 cc (11.4 mg, 0.36 mmol) for sea water. This difference has important implications for ocean life, as polar oceans support a much higher density of life due to their oxygen content.It is estimated that Algae produces about 73 to 87 percent of the net global production of oxygen, which makes it available to humans and other animals for respiration. Another secondary source of oxygen are trees, trees can absorb carbon dioxide at the rate of 26 pounds per year-especially young trees that are still growing-while releasing oxygen back into the air.Due to its electro negativity, oxygen forms chemical bonds with almost all other elements hence the original definition of oxidation. The only elements known to escape the possibility of oxidation are a few of the noble gases, and fluorine.The most familiar oxygen compound is water. Other well-known examples include compounds of carbon and oxygen, such as carbon dioxide (CO2 ( Oxygen )), alcohols (R-OH), carbonyls, (R-CO-H or R-CO-R), and carboxylic acids (R-COOH). Oxygenated radicals such as chlorates (ClO3−), perchlorates (ClO4−), chromates (CrO42−), dichromates (Cr2O72−), permanganates (MnO4−), and nitrates (NO3−) are strong oxidizing agents in and of themselves. Many metals bond with oxygen atoms, such as iron: resulting in iron(III) oxide (Fe2O3), commonly called rust.Ozone (O3) is formed by electrostatic discharge in the presence of molecular oxygen. A double oxygen molecule (O2 ) is known and is found as a minor component of liquid oxygen. Epoxides are ethers in which the oxygen atom is part of a ring of three atoms.One unexpected oxygen compound is dioxygen . It was noticed when a change in color when this compound was exposed to atmospheric air. Oxygen has seventeen known isotopes with atomic masses . Three are stable, 16O, 17O, and 18O, of which 16O is the most abundant (over 99.7%). The radioisotopes all have half-lives of less than three minutes. An atomic weight of 16 was assigned to oxygen prior to the definition of the unified atomic mass unit . Oxygen toxicityOxygen ,when taken through a hyperbaric chamber procedure or through oxygen tanks , can be toxic at elevated partial pressures. Since oxygen partial pressure is the fraction of oxygen times the total pressure, elevated partial pressures can occur either from high oxygen fraction in breathing gas, or from high breathing gas pressure, or a combination of both. Oxygen toxicity usually begins to occur at partial pressures more than 0.5 atmospheres, or 2.5 times the normal sea-level oxygen partial pressure of about 0.2 atmospheres or bars. This means that at sea-level pressures, mixtures containing less than 50% oxygen are essentially non-toxic. However in medical applications (such as in ventilation gas mixtures in hospital applications) mixtures containing more than 50% oxygen can be expected to show lung toxicity, causing slow damage to the lungs over periods of days, with the rate of damage rising rapidly from mixtures between 50% and 100% oxygen. On the other hand, breathing 100% oxygen in space applications (such as in some modern spacesuits, or in early spacecraft such as the Apollo spacecraft), causes no damage due to the low total pressures (30% to 33% sea-level) used. In the case of spacesuits, oxygen partial pressure in the breathing gas is typically about 0.30 bar (1.4 times normal), and oxygen partial pressure in the astronaut's blood (due to downward adjustments due to water vapor and CO2 ( Oxygen ) in the alveoli) is close to sea-level normal of 0.14 bar.In deep scuba diving and surface supplied diving and when using equipment which can provide high partial pressures of oxygen, such as rebreathers, oxygen toxicity to the lungs can occur, just as in medical applications. Due to the higher total pressures in these applications, the fraction of oxygen which produces lung damage may be considerably less than 50%. More importantly, under pressures higher than normal sea-level, a far more serious form of oxygen toxicity in the central nervous system may lead to generalized seizures. This form of oxygen toxicity usually occurs after several hours exposure to oxygen partial pressures over about 1.4 atmospheres with the time decreasing for higher pressures above this, and with great variation from person to person. At over three bars of oxygen partial pressure (15 times normal), seizures typically occur within minutes.Immune systems of higher organisms have long made use of reactive forms of oxygen which they produce. Air is the most common and only natural breathing gas. Other artificial gases, either pure gases or mixtures of gases, are used in breathing equipment and enclosed habitats such as SCUBA equipment, surface supplied diving equipment, recompression chambers, submarines, space suits, spacecraft and anaesthetic machines.Most breathing gases are a mixture of oxygen and one or more inert gases. All breathing gases are alternatives to air and have been developed to improve on the performance of air by reducing the risk of decompression sickness, reducing the duration of decompression stops, reducing nitrogen narcosis or allowing safer deep diving. Air is a mixture of oxygen, nitrogen, .Being simple to use, it is the most common diving gas. As its nitrogen component causes nitrogen narcosis it is considered to have a safe depth limit for most divers. Pure oxygen is mainly used to speed the shallow decompression stops at the end of a technical dive. It was much used in frogmen's rebreathers. Nitrox is a mixture of oxygen and air, and generally refers to mixtures which are more than 21% oxygen. It is mainly used instead of air to accelerate decompression or to decrease the risk of decompression sickness. Trimix is a mixture of oxygen, nitrogen and helium and is often used at depth in technical diving and commercial diving instead of air to reduce nitrogen narcosis. Heliox is a mixture of oxygen and helium and is often used in the deep phase of a commercial deep dive to eliminate nitrogen narcosis. Heliair is a form of trimix that is easily blended from helium and air without using pure oxygen. It always has a 21:79 ratio of oxygen to nitrogen; the balance of the mix is helium. Hydreliox is a mixture of oxygen, helium, and hydrogen and is used for dives below 130 metres in commercial diving. Neox is a mixture of oxygen and neon sometimes employed for in deep commercial diving. It is rarely used due to its cost. Also, DCS symptoms produced by neon ("neox bends") have a poor reputation, being widely reported to be more severe than those produced by an exactly equivalent dive-table and mix with helium. O2 ( Oxygen ) must be present in every breathing gas. This is because it is essential to the human body's metabolic process, which sustains life. If the body is deprived of oxygen for more than a few minutes, unconsciousness and death result. The tissues and organs within the body (notably the heart and brain) are damaged if deprived of oxygen for much longer than four minutes. After the body is deprived of oxygen all organs will shut down because of the lack of oxygen that is essential for human and animal life.The proportion of oxygen in a breathing gas determines the maximum operating depth, the deepest the mixture gas can safely be used: hypoxic mixes, strictly, contain less than 21% oxygen, although often a boundary of 16% is used, and are designed only to be breathed at depth as a "bottom gas" where the higher pressure increases the partial pressure of oxygen to a safe level. Normoxic mixes have the same proportion of oxygen as air, 21%. The maximum operating depth of a normoxic mix could be as shallow as 47 metres (155 feet). Trimix with between 17% and 21% oxygen is often described as normoxic because it contains a high enough proportion of oxygen to be safe to breathe at the surface. Hyperoxic mixes have a more oxygen than 21%. Enriched Air Nitrox (EAN) is a typical hyperoxic breathing gas. The minimum safe partial pressure of oxygen in a breathing gas is commonly held to be 16 kPa (0.16 bar). Below this partial pressure the diver may be at risk of unconsciousness and death due to hypoxia, depending on factors including individual physiology and level of exertion. When a hypoxic mix is breathed in shallow water it may not have a high enough ppO2 ( Oxygen ) to keep the diver conscious. For this reason normoxic or hyperoxic "travel gases" are used at medium depth between the "bottom" and "decompression" phases of the dive.The maximum safe partial pressure of oxygen in a breathing gas depends on exposure time, the level of exercise and the security of the breathing equipment being used. It is typically between 100 kPa (1 bar) and 160 kPa (1.6 bar) but for dives of less than three hours is commonly considered to be 140 kPa (1.4 bar), although the U.S. Navy has been known to authorize dives with a partial oxygen pressure of as much as 180 kPa (1.8 bar). At high partial pressures or longer exposures, the diver risks oxygen toxicity including a seizure similar to an epileptic fit. Each breathing gas has a maximum operating depth that is determined by its oxygen content. Oxygen analyzers are used to measure the partial pressure of oxygen in the gas mix. Filling a diving cylinder with pure oxygen costs around five times more than filling it with compressed air. As oxygen supports combustion and causes rust in diving cylinders, it should be handled with respect when gas blending. Oxygen is obtained by fractional distillation of liquid air."Divox" is oxygen. In the Netherlands, pure oxygen for breathing purposes is regarded as medicinal as opposed to industrial oxygen, such as that used in welding, and is only available on medical prescription. The diving industry "created" Divox and registered it as a trademark to circumvent the strict rules concerning medicinal oxygen thus making it easier for (recreational) scuba divers to obtain oxygen for blending their breathing gas. Nitrogen (N2) is an inert gas and the main component of air, the cheapest and most common breathing gas used for diving. It causes nitrogen narcosis in the diver, so its use is limited to shallower dives. Nitrogen can cause decompression sickness. Equivalent air depth is often used to help design a breathing gas mix by determining the maximum nitrogen content for a particular depth of dive. Many divers find that the level of narcosis caused by a 30-metre (100-foot) dive, whilst breathing air, is a comfortable maximum. The partial pressure of nitrogen at this depth on air is 316 kPa (3.16 bar) (Fraction of nitrogen x absolute pressure = 0.79 x 400 kPa). So, what fraction of nitrogen would cause the same narcosis at 60 metres? The answer is 45% nitrogen. Helium (He) is an inert gas that is less narcotic than nitrogen at equivalent pressure (in fact there is no evidence for any narcosis from helium at all), so it is more suitable for deeper dives than nitrogen. Helium is equally able to cause decompression sickness. At high pressures, helium also causes High Pressure Nervous Syndrome, which is a CNS irritation syndrome which is in some ways opposite to narcosis. Helium fills typically cost ten times more than an equivalent air fill. Helium is not very suitable for dry suit inflation due to its poor thermal insulation properties — helium is a very good conductor of heat (compared to air which is a rather poor, making it more of an insulator). Helium's low molecular weight (monoatomic MW=4, compared with diatomic nitrogen MW=28) increases the pitch of the breather's voice, which may impede communication. This is because the speed of sound is faster in a lower molecular weight gas, which increases the resonant frequency of the vocal cords. Helium leaks from damaged or faulty valves more readily than other gases because atoms of helium are smaller allowing them to pass through smaller gaps in seals. Helium is found in significant amounts only in natural gas, from which it is extracted at low temperatures by fractional distillation. ydrogen (H2) has been used in deep diving gas mixes but is very explosive when mixed with more than about 4 to 5% oxygen (such as the oxygen found in breathing gas). This limits use of hydrogen to deep dives and imposes complicated protocols to insure that oxygen is cleared from the lungs, the blood stream and the breathing equipment before breathing hydrogen starts. Like helium, it increases the pitch of the diver's voice. Many gases are not suitable for use in diving breathing gases. Here is an incomplete list of gases commonly present in a diving environment: Argon (Ar) is an inert gas that is more narcotic than nitrogen, so is not suitable as a diving breathing gas. Argon is more expensive than air or oxygen, but considerably less expensive than helium. Carbon dioxide (CO2 ( Oxygen )) is produced by the metabolism in the human body and causes carbon dioxide poisoning. Carbon monoxide (CO) is produced by incomplete combustion.Carbon monoxide poisoning nternal combustion engine exhaust gas containing CO in the air being drawn into a diving air compressor. CO in the intake air cannot be stopped by any filter. All internal combustion engines running on petroleum fuels contain some CO, and this is a particular problem on boats, where the intake of the compressor cannot be arbitrarily moved as far as desired from the engine and compressor exhausts. Heating of lubricants inside the compressor may vaporize them sufficiently to be available to a compressor intake or intake system line. Hydrocarbons (CxHy) are present in compressor lubricants and fuels. They can enter diving cylinders as a result of contamination, leaks, or due to incomplete combustion near the air intake. They can act as a fuel in combustion increasing the risk of explosion, especially in high-oxygen gas mixtures. Inhaling oil mist can damage the lungs and ultimately cause the lungs to degenerate with severe emphysema. The process of compressing gas into a diving cylinder removes moisture from the gas. This is good for corrosion prevention in the cylinder but means that the diver inhales very dry gas. The dry gas extracts moisture from the divers lungs while underwater contributing to dehydration, which is also thought to be a predisposing risk factor of decompression sickness. It is also uncomfortable, causing a dry mouth and throat and making the diver thirsty. This problem is reduced in rebreathers because the soda lime reaction to remove carbon dioxide puts moisture back into the breathing gas. In hot, tropical climates, open circuit diving can accelerate heat exhaustion because of dehydration.Divers find it difficult to detect most gases that are likely to be present in diving cylinders because they are colorless, odorless and tasteless. Electronic sensors exist for some gases, such as oxygen analyzers, helium analyzer, carbon monoxide detectors and carbon dioxide detectors. Oxygen analysers are commonly found underwater in rebreathers. Oxygen and helium analysers are often used on the surface during gas blending to determine the percentage of oxygen or helium in a breathing gas mix. Chemical and other types of gas detection methods are not often used in recreational diving.What is HypoxiaHypoxia is a pathological condition in which the body as a whole or region of the body is deprived of adequate oxygen supply. Hypoxia in which there is complete deprivation of oxygen supply is referred to as anoxia. Hypoxia is distinguished from apoxemia. Apoxemia is an abnormally low partial pressure of oxygen in arterial blood A frequent error is to use the term hypoxemia to mean low oxygen content in arterial blood. It is possible to have a low oxygen content (due to anemia) . Generalised hypoxia occurs in healthy people when they ascend to high altitude, where it causes altitude sickness, and the potentially fatal complications of altitude sickness, high altitude pulmonary oedema (HAPE) and high altitude cerebral oedema (HACE). Hypoxia also occurs in healthy individuals when breathing mixtures of gases with a low oxygen content, for example while diving underwater, especially with closed-circuit rebreather systems that control the amount of oxygen in the air breathed in. Altitude training uses mild hypoxia to increase the concentration of red blood cells in the body for increased athletic performance.Symptoms of hypoxiaSymptoms of generalized hypoxia depend on its severity and speed of onset. In the case of altitude sickness, where hypoxia develops gradually, the symptoms include headaches, fatigue, shortness of breath, and nausea. In severe hypoxia, or hypoxia of very rapid onset, changes in levels of consciousness, seizures, coma and death occur. Severe hypoxia induces a blue discolouration of the skin, called cyanosis (haemoglobin is blue when it is not bound to oxygen (deoxyhaemoglobin), as opposed to the rich red color that it has when bound to oxygen (oxyhaemoglobin)). In cases where the oxygen is displaced by another molecule, such as carbon monoxide, the skin may be 'cherry red' instead of cyanotic.Types of hypoxiaHypoxic hypoxia is a generalized hypoxia, an inadequate supply of oxygen to the body as a whole. The term "hypoxic hypoxia" refers to the fact that hypoxia occurs as a consequence of low partial pressure of oxygen in arterial blood, in contrast to the other causes of hypoxia that follow, in which the partial pressure of oxygen in arterial blood is normal. Hypoxic hypoxia may be due to: Low partial pressure of atmospheric oxygen such as found at high altitude or by replacement of oxygen in the breathing mix either accidentally as in the modified atmosphere of a sewer or intentionally as in the recreational use of nitrous oxide. Either Sleep apnea or Hypopnea causing a decrease in oxygen saturation of the blood. Inadequate pulmonary ventilation (chronic obstructive pulmonary disease or respiratory arrest). Shunts in the pulmonary circulation or a right-to-left shunt in the heart. Shunts can be caused by collapsed alveoli that are still perfused or a block in ventilation to an area of the lung. Whatever the mechanism, blood meant for the pulmonary system is not ventilated and so no gas exchange occurs (the ventilation/perfusion ratio is zero). Normal anatomical shunt occurs in everyone, because of the Thebesian vessels which empty into the left ventricle and the bronchial circulation which supplies the bronchi with oxygen. Anemic hypoxia in which arterial oxygen pressure is normal, but total oxygen content of the blood is reduced. Hypemic hypoxia when there is an inability of the blood to deliver oxygen to target tissues. Carbon monoxide poisoning which inhibits the ability of haemoglobin to release the oxygen bound to it. Methaemoglobinaemia in which an abnormal version of haemoglobin accumulates in the blood Histotoxic hypoxia in which quantity of oxygen reaching the cells is normal, but the cells are unable to effectively use the oxygen due to disabled oxidative phosphorylation enzymes. Ischemic, or stagnant hypoxia in which there is a local restriction in the flow of otherwise well-oxygenated blood. The oxygen supplied to the region of the body is then insufficient for its needs. Examples are cerebral ischemia, ischemic heart disease and Intrauterine hypoxia, which is an unchallenged cause of perinatal death. After mixing with water vapor and expired CO2 ( Oxygen ) in the lungs, oxygen diffuses down a pressure gradient to enter arterial blood around where its partial pressure is 100mmHg . Arterial blood flow delivers oxygen to the peripheral tissues, where it again diffuses down a pressure gradient into the cells and into their mitochondria. These bacterial like cytoplasmic structures strip hydrogen from fuels (glucose, fats and some amino acids) to burn with oxygen to form water. Released energy (originally from the sun and photosynthesis) is stored as ATP, to be later used for energy requiring metabolism. The fuel's carbon is oxidized to CO2 ( Oxygen ), which diffuses down its partial pressure gradient out of the cells into venous blood to finally be exhaled by the lungs. Experimentally, oxygen diffusion becomes rate limiting (and lethal) when arterial oxygen partial pressure falls to 40mmHg or below.If oxygen delivery to cells is insufficient for the demand (hypoxia), hydrogen will be shifted to pyruvic acid converting it to lactic acid. This temporary measure (anaerobic metabolism) allows small amounts of energy to be produced. Lactic acid build up in tissues and blood is a sign of inadequate mitochondrial oxygenation, which may be due to hypoxemia, poor blood flow (e.g. shock) or a combination of both. If severe or prolonged it could lead to cell death. In most tissues of the body, the response to hypoxia is vasodilation. By widening the blood vessels, the tissue allows greater perfusion.By contrast, in the lungs, the response to hypoxia is vasoconstriction. This is known as "Hypoxic pulmonary vasoconstriction", or "HPV".Hypoxia or Oxygen depletion ypoxia or oxygen depletion is a phenomenon that occurs in aquatic environments as dissolved oxygen (DO; molecular oxygen dissolved in the water) becomes reduced in concentration to a point detrimental to aquatic organisms living in the system. Dissolved oxygen is typically expressed as a percentage of the oxygen that would dissolve in the water at the prevailing temperature and salinity (both of which affect the solubility of oxygen in water; see oxygen saturation and underwater). An aquatic system lacking dissolved oxygen (0% saturation) is termed anaerobic, reducing, or anoxic; a system with low DO concentration—in the range between 1 and 30% DO saturation—is called hypoxic. Most fish cannot live below 30% DO saturation. A "healthy" aquatic environment should seldom experience DO less than 80%.Oxygen depletion could be the result of a number of factors including natural ones, but is of most concern as a consequence of pollution and as an outcome of a process known as eutrophication. Where plant nutrients enter a river, lake, or ocean, phytoplankton blooms are encouraged. While phytoplankton, through photosynthesis, will raise DO saturation during daylight hours, the dense population of a bloom reduces DO saturation during the night. When phytoplankton cells die, they sink towards the bottom and are decomposed by bacteria, a process that further reduces DO in the water column. If oxygen depletion progresses to hypoxia, fish kills can occur and invertebrates like worms and clams on the bottom may be killed as well.Natural occurrences of hypoxia have been observed. Water flowing from a river into the sea is less dense than salt water. When this water does not mix with the underlying saline water, the oxygen concentration in the bottom layer may become low enough for hypoxia to occur. Hypoxia is particularly problematic in shallow waters of semi-enclosed bodies of water like the Waddenzee or the Gulf of Mexico where land runoff is substantial. In these areas, a so-called "dead zone" can be created.In a very short time the oxygen saturation can drop to zero when offshore blowing winds drive surface water out and anoxic depth water rises up. At the same time a decline in temperature and a rise in salinity is observed . New approaches of long term monitoring of oxygen regime in the ocean observe online the behavior of fish which changes drastically under reduced oxygen saturations and already at very low levels of water pollution.The amount of dissolved oxygen is an indicator of the cleanliness of water. A high level of dissolved oxygen in a water sample shows that the water is clean and unpolluted.