<unk> is a condition in which the body or a region of the body is deprived of adequate oxygen supply at the tissue level.<unk> may be classified as either generalized, affecting the whole body, or local, affecting a region of the body. Although<unk> is often a pathological condition, variations in arterial oxygen concentrations can be part of the normal physiology, for example, during hypoventilation training or strenuous physical exercise.<unk> differs from hypoxemia and anoxemia in that<unk> refers to a state in which oxygen supply is insufficient, whereas hypoxemia and anoxemia refer specifically to states that have low or zero arterial oxygen supply.<unk> in which there is complete deprivation of oxygen supply is referred to as anoxia. Generalized<unk> occurs in healthy people when they ascend to high altitude, where it causes altitude sickness leading to potentially fatal complications: high altitude pulmonary edema (HAPE) and high altitude cerebral edema (HACE).<unk> also occurs in healthy individuals when breathing mixtures of gases with a low oxygen content, e.g. while diving underwater especially when using closed-circuit rebreather systems that control the amount of oxygen in the supplied air. Mild, non-damaging intermittent<unk> is used intentionally during altitude training to develop an athletic performance adaptation at both the systemic and cellular level. In acute or silent<unk>  a person's oxygen level in blood cells and tissue can drop without any initial warning, even though the individual's chest x-ray shows diffuse pneumonia with an oxygen level below normal. Doctors report cases of silent<unk> with COVID-19 patients who did not experience shortness of breath or coughing until their oxygen levels had plummeted to such a degree that the patients risked acute respiratory distress (ARDS) and organ failure. In a New York Times opinion piece (April 20, 2020), emergency room doctor Richard Levitan reports: "A vast majority of Covid pneumonia patients I met had remarkably low oxygen saturations at triage—seemingly incompatible with life—but they were using their cellphones as we put them on monitors."<unk> is a common complication of preterm birth in newborn infants. Because the lungs develop late in pregnancy, premature infants frequently possess underdeveloped lungs. To improve lung function, doctors frequently place infants at risk of<unk> inside incubators (also known as humidicribs) that provide warmth, humidity, and oxygen. More serious cases are treated with CPAP. The 2019 Nobel Prize in Physiology or Medicine was awarded to William G. Kaelin Jr., Sir Peter J. Ratcliffe, and Gregg L. Semenza in recognition of their discovery of cellular mechanisms to sense and adapt to different oxygen concentrations, establishing a basis for how oxygen levels affect physiological function.
Hypoxia 
Traumatic cardiac arrest
Anoxia
Hyperoxia
Acidosis
Cerebral hypoxia
Pseudohypoxia
Nitrogen narcosis