Asphyxia or asphyxiation is a condition of severely deficient supply of oxygen to the body that arises from abnormal breathing. An example of asphyxia is choking. Asphyxia causes generalized hypoxia, which affects primarily the tissues and organs. There are many circumstances that can induce asphyxia, all of which are characterized by an inability of an individual to acquire sufficient oxygen through breathing for an extended period of time. Asphyxia can cause coma or death.

In 2015 about 9.8 million cases of unintentional suffocation occurred which resulted in 35,600 deaths. The word asphyxia is from Ancient Greek "without" and , "squeeze" (throb of heart).

Situations that can cause asphyxia include but are not limited to: the constriction or obstruction of airways, such as from asthma, laryngospasm, or simple blockage from the presence of foreign materials; from being in environments where oxygen is not readily accessible: such as underwater, in a low oxygen atmosphere, or in a vacuum; environments where sufficiently oxygenated air is present, but cannot be adequately breathed because of air contamination such as excessive smoke.

Other causes of oxygen deficiency include

but are not limited to:

- Acute respiratory distress syndrome

- Carbon monoxide inhalation, such as that from a car exhaust and the smoke's emission from a lighted cigarette: carbon monoxide has a higher affinity than oxygen to the hemoglobin in the blood's red blood corpuscles, bonding with it tenaciously, and, in the process, displacing oxygen and preventing the blood from transporting oxygen around the body

- Contact with certain chemicals, including pulmonary agents (such as phosgene) and blood agents (such as hydrogen cyanide)

- Drowning

- Drug overdose

- Exposure to extreme low pressure or vacuum to the pattern (see space exposure)

- Hanging, specifically suspension or short drop hanging

- Self-induced hypocapnia by hyperventilation, as in shallow water or deep water blackout and the choking game

- Inert gas asphyxiation

- Congenital central hypoventilation syndrome, or primary alveolar hypoventilation, a disorder of the autonomic nervous system in which a patient must consciously breathe; although it is often said that persons with this disease will die if they fall asleep, this is not usually the case

- Respiratory diseases

- Sleep apnea

- A seizure which stops breathing activity

- Strangling

- Breaking the wind pipe.

- Prolonged exposure to chlorine gas

There are several terms which were in general use, but are no longer recommended.

Drowning is most often quick and unspectacular. Its media depictions as a loud, violent struggle have much more in common with distressed non-swimmers, who may well drown but have not yet begun to do so. In particular, an asphyxiating person is seldom able to call for help. The instinctive drowning response covers many signs or behaviors associated with drowning or near-drowning:

- Head low in the water, mouth at water level

- Head tilted back with mouth open

- Eyes glassy and empty, unable to focus

- Eyes open, with fear evident on the face

- Hyperventilating or gasping

- Trying to swim in a particular direction but not making headway

- Trying to roll over on the back to float

- Uncontrollable movement of arms and legs, rarely out of the water.

Frank Pia, a lifeguard and researcher of rescue techniques and drowning, notes that drowning begins at the point a person is unable to keep their mouth above water; inhalation of water takes place at a later stage. Most people demonstrating the instinctive drowning response do not show obvious prior evidence of distress.

The symptoms of generalized hypoxia depend on its severity and acceleration of onset.

In the case of altitude sickness, where hypoxia develops gradually, the symptoms include fatigue, numbness / tingling of extremities, nausea, and anoxia. In severe hypoxia, or hypoxia of very rapid onset, ataxia, confusion / disorientation / hallucinations / behavioral change, severe headaches / reduced level of consciousness, papilloedema, breathlessness, pallor, tachycardia, and pulmonary hypertension eventually leading to the late signs cyanosis, slow heart rate / cor pulmonale, and low blood pressure followed by death.

Because hemoglobin is a darker red when it is not bound to oxygen (deoxyhemoglobin), as opposed to the rich red color that it has when bound to oxygen (oxyhemoglobin), when seen through the skin it has an increased tendency to reflect blue light back to the eye. In cases where the oxygen is displaced by another molecule, such as carbon monoxide, the skin may appear 'cherry red' instead of cyanotic. Hypoxia can cause premature birth, and injure the liver, among other deleterious effects.

If tissue is not being perfused properly, it may feel cold and appear pale; if severe, hypoxia can result in cyanosis, a blue discoloration of the skin. If hypoxia is very severe, a tissue may eventually become gangrenous.

Extreme pain may also be felt at or around the site.

Inert gas asphyxiation is a form of asphyxiation which results from breathing a physiologically inert gas in the absence of oxygen, or a low amount of oxygen, rather than atmospheric air (which is largely composed of nitrogen and oxygen). Examples of physiologically inert gases, which have caused accidental or deliberate death by this mechanism, are: argon, helium, nitrogen and methane. The term "physiologically inert" is used to indicate a gas which has no toxic or anesthetic properties and does not act upon the heart or hemoglobin. Instead, the gas acts as a simple diluent to reduce oxygen concentration in inspired gas and blood to dangerously low levels, thereby eventually depriving all cells in the body of oxygen.

According to the U.S. Chemical Safety and Hazard Investigation Board, in humans, "breathing an oxygen deficient atmosphere can have serious and immediate effects, including unconsciousness after only one or two breaths. The exposed person has no warning and cannot sense that the oxygen level is too low." In the US, at least 80 people died due to accidental nitrogen asphyxiation between 1992 and 2002. Hazards with inert gases and the risks of asphyxiation are well established.

An occasional cause of accidental death in humans, inert gas asphyxia with gases including helium, nitrogen, methane, and argon, has been used as a suicide method. Inert gas asphyxia has been advocated by proponents of euthanasia, using a gas-retaining plastic hood device colloquially referred to as a suicide bag.

Nitrogen asphyxiation has been suggested by a number of lawmakers and other advocates as a more humane way to carry out capital punishment. In April 2015, the Oklahoma Governor Mary Fallin signed a bill authorizing nitrogen asphyxiation as an alternative execution method in cases where the state's preferred method of lethal injection was not available as an option.

The brain requires approximately 3.3 ml of oxygen per 100 g of brain tissue per minute. Initially the body responds to lowered blood oxygen by redirecting blood to the brain and increasing cerebral blood flow. Blood flow may increase up to twice the normal flow but no more. If the increased blood flow is sufficient to supply the brain's oxygen needs then no symptoms will result.

However, if blood flow cannot be increased or if doubled blood flow does not correct the problem, symptoms of cerebral hypoxia will begin to appear. Mild symptoms include difficulties with complex learning tasks and reductions in short-term memory. If oxygen deprivation continues, cognitive disturbances, and decreased motor control will result. The skin may also appear bluish (cyanosis) and heart rate increases. Continued oxygen deprivation results in fainting, long-term loss of consciousness, coma, seizures, cessation of brain stem reflexes, and brain death.

Objective measurements of the severity of cerebral hypoxia depend on the cause. Blood oxygen saturation may be used for hypoxic hypoxia, but is generally meaningless in other forms of hypoxia. In hypoxic hypoxia 95–100% saturation is considered normal; 91–94% is considered mild and 86–90% moderate. Anything below 86% is considered severe.

It should be noted that cerebral hypoxia refers to oxygen levels in brain tissue, not blood. Blood oxygenation will usually appear normal in cases of hypemic, ischemic, and hystoxic cerebral hypoxia. Even in hypoxic hypoxia blood measures are only an approximate guide; the oxygen level in the brain tissue will depend on how the body deals with the reduced oxygen content of the blood.

Cerebral hypoxia can be caused by any event that severely interferes with the brain's ability to receive or process oxygen. This event may be internal or external to the body. Mild and moderate forms of cerebral hypoxia may be caused by various diseases that interfere with breathing and blood oxygenation. Severe asthma and various sorts of anemia can cause some degree of diffuse cerebral hypoxia. Other causes include status epilepticus, work in nitrogen-rich environments, ascent from a deep-water dive, flying at high altitudes in an unpressurized cabin without supplemental oxygen, and intense exercise at high altitudes prior to acclimatization.

Severe cerebral hypoxia and anoxia is usually caused by traumatic events such as choking, drowning, strangulation, smoke inhalation, drug overdoses, crushing of the trachea, status asthmaticus, and shock. It is also recreationally self-induced in the fainting game and in erotic asphyxiation.

- Transient ischemic attack (TIA), is often referred to as a "mini-stroke". The American Heart Association and American Stroke Association (AHA/ASA) refined the definition of transient ischemic attack. TIA is now defined as a transient episode of neurologic dysfunction caused by focal brain, spinal cord, or retinal ischemia, without acute infarction. The symptoms of a TIA can resolve within a few minutes, unlike a stroke. TIAs share the same underlying etiology as strokes; a disruption of cerebral blood flow. TIAs and strokes present with the same symptoms such as contralateral paralysis (opposite side of body from affected brain hemisphere), or sudden weakness or numbness. A TIA may cause sudden dimming or loss of vision, aphasia, slurred speech, and mental confusion. The symptoms of a TIA typically resolve within 24 hours, unlike a stroke. Brain injury may still occur in a TIA lasting only a few minutes. Having a TIA is a risk factor for eventually having a stroke.

- Silent stroke is a stroke which does not have any outward symptoms, and the patient is typically unaware they have suffered a stroke. Despite its lack of identifiable symptoms, a silent stroke still causes brain damage and places the patient at increased risk for a major stroke in the future. In a broad study in 1998, more than 11 million people were estimated to have experienced a stroke in the United States. Approximately 770,000 of these strokes were symptomatic and 11 million were first-ever silent MRI infarcts or hemorrhages. Silent strokes typically cause lesions which are detected via the use of neuroimaging such as fMRI. The risk of silent stroke increases with age but may also affect younger adults. Women appear to be at increased risk for silent stroke, with hypertension and current cigarette smoking being predisposing factors.

When humans breathe in an asphyxiant gas, such as pure nitrogen, helium, neon, argon, sulfur hexafluoride, methane, or any other physiologically inert gas(es), they exhale carbon dioxide without re-supplying oxygen. Physiologically inert gases (those that have no toxic effect, but merely dilute oxygen) are generally free of odor and taste. As such, the human subject detects little abnormal sensation as the oxygen level falls. This leads to asphyxiation (death from lack of oxygen) without the painful and traumatic feeling of suffocation (the hypercapnic alarm response, which in humans arises mostly from carbon dioxide levels rising), or the side effects of poisoning. In scuba diving rebreather accidents, there is often little sensation but euphoria—however, a slow decrease in oxygen breathing gas content has effects which are quite variable. By contrast, suddenly breathing pure inert gas causes oxygen levels in the blood to fall precipitously, and may lead to unconsciousness in only a few breaths, with no symptoms at all.

Some animal species are better equipped than humans to detect hypoxia, and these species are more uncomfortable in low-oxygen environments that result from inert gas exposure.

Choking (also known as foreign body airway obstruction) is a life-threatening medical emergency characterized by the blockage of air passage into the lungs secondary to the inhalation or ingestion of food or another object.

Choking is caused by a mechanical obstruction of the airway that prevents normal breathing. This obstruction can be partial (allowing some air passage into the lungs) or complete (no air passage into the lungs). The disruption of normal breathing by choking deprives oxygen delivery to the body, resulting in asphyxia. Although oxygen stored in the blood and lungs can keep a person alive for several minutes after breathing stops, this sequence of events is potentially fatal. Choking was the fourth most common cause of unintentional injury-related death in the US in 2011.

Deaths from choking most often occur in the very young (children under 1 years old) and in the elderly (adults over 75 years). Obstruction of the airway can occur at the level of the pharynx or the trachea. Foods that can adapt their shape to that of the pharynx (such as bananas, marshmallows, or gelatinous candies) can be a danger not just for children but for persons of any age.

Choking is one type of airway obstruction, which includes any blockage of the air-conducting passages, including blockage due to tumors, swelling of the airway tissues, and compression of the laryngopharynx, larynx or vertebrate trachea in strangulation.

Intrauterine hypoxia occurs when the fetus is deprived of an adequate supply of oxygen. It may be due to a variety of reasons such as prolapse or occlusion of the umbilical cord, placental infarction and maternal smoking. Intrauterine growth restriction (IUGR) may cause or be the result of hypoxia. Intrauterine hypoxia can cause cellular damage that occurs within the central nervous system (the brain and spinal cord). This results in an increased mortality rate, including an increased risk of sudden infant death syndrome (SIDS). Oxygen deprivation in the fetus and neonate have been implicated as either a primary or as a contributing risk factor in numerous neurological and neuropsychiatric disorders such as epilepsy, ADHD, eating disorders and cerebral palsy.

Choking is caused by an object from outside the body, also called a foreign body, blocking the airway. The object can block the upper or lower airway passages. The airway obstruction is usually partial but can also be complete.

Among children, the most common causes of choking are food, coins, toys, and balloons. In one study, peanuts were the most common object found in the airway of children evaluated for suspected foreign body aspiration. Foods that pose a high risk of choking include hot dogs, hard candy, nuts, seeds, whole grapes, raw carrots, apples, popcorn, peanut butter, marshmallows, chewing gum, and sausages. The most common cause of choking death in children is latex balloons. Small, round non-food objects such as balls, marbles, toys, and toy parts are also associated with a high risk of choking death because of their potential to completely block a child's airway.

Children younger than age three are especially at risk of choking because they explore the environment by putting objects in their mouth. Also, young children are still developing the ability to chew food completely. Molar teeth, which come in around 1.5 years of age, are necessary for grinding food. Even after molar teeth are present, children continue developing the ability to chew food completely and swallow throughout early childhood. In addition, a child's airway is smaller in diameter than an adult's airway, which means that smaller objects can cause an airway obstruction in children. Infants and young children generate a less forceful cough than adults, so coughing may not be as effective in relieving an airway obstruction. Finally, children with neuromuscular disorders, developmental delay, traumatic brain injury, and other conditions that affect swallowing are at an increased risk of choking.

In adults, choking often involves food blocking the airway. Risk factors include using alcohol or sedatives, undergoing a procedure involving the oral cavity or pharynx, wearing oral appliances, or having a medical condition that causes difficulty swallowing or impairs the cough reflex. Conditions that can cause difficulty swallowing and/or impaired coughing include neurologic conditions such as strokes, Alzheimer disease, or Parkinson disease. In older adults, risk factors also include living alone, wearing dentures, and having difficulty swallowing.

Children and adults with neurologic, cognitive, or psychiatric disorders may experience a delay in diagnosis because there may not be a known history of a foreign body entering the airway.

Perinatal asphyxia, neonatal asphyxia or birth asphyxia is the medical condition resulting from deprivation of oxygen to a newborn infant that lasts long enough during the birth process to cause physical harm, usually to the brain. Hypoxic damage can occur to most of the infant's organs (heart, lungs, liver, gut, kidneys), but brain damage is of most concern and perhaps the least likely to quickly or completely heal. In more pronounced cases, an infant will survive, but with damage to the brain manifested as either mental, such as developmental delay or intellectual disability, or physical, such as spasticity.

It results most commonly from a drop in maternal blood pressure or some other substantial interference with blood flow to the infant's brain during delivery. This can occur due to inadequate circulation or perfusion, impaired respiratory effort, or inadequate ventilation. Perinatal asphyxia happens in 2 to 10 per 1000 newborns that are born at term, and more for those that are born prematurely. WHO estimates that 4 million neonatal deaths occur yearly due to birth asphyxia, representing 38% of deaths of children under 5 years of age.

Perinatal asphyxia can be the cause of hypoxic ischemic encephalopathy or intraventricular hemorrhage, especially in preterm births. An infant suffering severe perinatal asphyxia usually has poor color (cyanosis), perfusion, responsiveness, muscle tone, and respiratory effort, as reflected in a low 5 minute Apgar score. Extreme degrees of asphyxia can cause cardiac arrest and death. If resuscitation is successful, the infant is usually transferred to a neonatal intensive care unit.

There has long been a scientific debate over whether newborn infants with asphyxia should be resuscitated with 100% oxygen or normal air. It has been demonstrated that high concentrations of oxygen lead to generation of oxygen free radicals, which have a role in reperfusion injury after asphyxia. Research by Ola Didrik Saugstad and others led to new international guidelines on newborn resuscitation in 2010, recommending the use of normal air instead of 100% oxygen.

There is considerable controversy over the diagnosis of birth asphyxia due to medicolegal reasons. Because of its lack of precision, the term is eschewed in modern obstetrics.

The most obvious sign that meconium has been passed during or before labor is the greenish or yellowish appearance of the amniotic fluid. The infant's skin, umbilical cord, or nailbeds may be stained green if the meconium was passed a considerable amount of time before birth. These symptoms alone do not necessarily indicate that the baby has inhaled in the fluid by gasping in utero or after birth. After birth, rapid or labored breathing, cyanosis, slow heartbeat, a barrel-shaped chest or low Apgar score are all signs of the syndrome. Inhalation can be confirmed by one or more tests such as using a stethoscope to listen for abnormal lung sounds (diffuse 'wet' crackles and rhonchi), performing blood gas tests to confirm a severe loss of lung function (respiratory acidosis as a consequence of hypercapnia), and using chest X-rays to look for patchy or streaked areas on the lungs. Infants who have inhaled meconium may develop respiratory distress syndrome often requiring ventilatory support. Complications of MAS include pneumothorax and persistent pulmonary hypertension of the newborn.

There are various causes for intrauterine hypoxia (IH). The most preventable cause is maternal smoking. Cigarette smoking by expectant mothers has been shown to have a wide variety of deleterious effects on the developing fetus. Among the negative effects are carbon monoxide induced tissue hypoxia and placental insufficiency which causes a reduction in blood flow from the uterus to the placenta thereby reducing the availability of oxygenated blood to the fetus. Placental insufficiency as a result of smoking has been shown to have a causal effect in the development of pre-eclampsia. While some previous studies have suggested that carbon monoxide from cigarette smoke may have a protective effect against preeclampsia, a recent study conducted by the Genetics of Pre-Eclampsia Consortium (GOPEC) in the United Kingdom found that smokers were five times more likely to develop pre-eclampsia.

Nicotine alone has been shown to be a teratogen which affects the autonomic nervous system, leading to increased susceptibility to hypoxia-induced brain damage.

Maternal anemia in which smoking has also been implicated is another factor associated with IH/BA. Smoking by expectant mothers causes a decrease in maternal nucleated red blood cells (NRBC), thereby reducing the amount of red blood cells available for oxygen transport.

The perinatal brain injury occurring as a result of birth asphyxia, manifesting within 48 hours of birth, is a form of hypoxic ischemic encephalopathy.

Meconium aspiration syndrome (MAS) also known as neonatal aspiration of meconium is a medical condition affecting newborn infants. It occurs when meconium is present in their lungs during or before delivery. Meconium is the first stool of an infant, composed of materials ingested during the time the infant spends in the uterus.

Meconium is normally stored in the infant's intestines until after birth, but sometimes (often in response to fetal distress and hypoxia) it is expelled into the amniotic fluid prior to birth, or during labor. If the baby then inhales the contaminated fluid, respiratory problems may occur.

Transient tachypnea of the newborn (TTN, TTNB, or "transitory tachypnea of newborn") is a respiratory problem that can be seen in the newborn shortly after delivery. Amongst causes of respiratory distress in term neonates, it is the most common. It consists of a period of rapid breathing (higher than the normal range of 30-60 times per minute). It is likely due to amniotic fluid remaining in the lungs after birth. Usually, this condition resolves over 24–48 hours. Treatment is supportive and may include supplemental oxygen and antibiotics. The chest x-ray shows hyperinflation of the lungs including prominent pulmonary vascular markings, flattening of the diaphragm, and fluid in the horizontal fissure of the right lung.

Strangling is compression of the neck that may lead to unconsciousness or death by causing an increasingly hypoxic state in the brain. Fatal strangling typically occurs in cases of violence, accidents, and is one of two main ways that hanging may cause death (alongside breaking the victim's neck).

Strangling does not have to be fatal; limited or interrupted strangling is practised in erotic asphyxia, in the choking game, and is an important technique in many combat sports and self-defence systems.

Strangling can be divided into three general types according to the mechanism used:

- Hanging—Suspension from a cord wound around the neck

- Ligature strangulation—Strangulation without suspension using some form of cord-like object called a garrote

- Manual strangulation—Strangulation using the fingers or other extremity

TTN is a diagnosis of exclusion as it is a benign condition that can have symptoms and signs similar to more serious conditions, such as respiratory distress syndrome. A chest X-ray may show a radiopaque line - fluid - in the horizontal fissure of the right lung, fluid infiltrate throughout alveoli or fluid in individual lung lobes. The lungs may also appear hyperinflated.

Strangling involves one or several mechanisms that interfere with the normal flow of oxygen into the brain:

- Compression of the carotid arteries or jugular veins—causing cerebral ischemia.

- Compression of the laryngopharynx, larynx, or trachea—causing asphyxia.

- Stimulation of the carotid sinus reflex—causing bradycardia, hypotension, or both.

Depending on the particular method of strangulation, one or several of these typically occur in combination; vascular obstruction is usually the main mechanism. Complete obstruction of blood flow to the brain is associated with irreversible neurological damage and death, but during strangulation there is still unimpeded blood flow in the vertebral arteries. Estimates have been made that significant occlusion of the carotid arteries and jugular veins occurs with a pressure of around , while the trachea demands six times more at approximately .

As in all cases of strangulation, the rapidity of death can be affected by the susceptibility to carotid sinus stimulation. Carotid sinus reflex death is sometimes considered a mechanism of death in cases of strangulation, but it remains highly disputed. The reported time from application to unconsciousness varies from 7–14 seconds if effectively applied to one minute in other cases, with death occurring minutes after unconsciousness.

Traumatic asphyxia, or Perthes' syndrome, is a medical emergency caused by an intense compression of the thoracic cavity, causing venous back-flow from the right side of the heart into the veins of the neck and the brain.

In neonates born at or beyond 35 weeks, neonatal encephalopathy may present itself as the following symptoms:

- Reduced level of consciousness

- Seizures (which peak at 48 hours)

- Difficulty initiating and maintaining respiration

- Depression of tone and reflexes

Traumatic asphyxia is characterized by cyanosis in the upper extremities, neck, and head as well as petechiae in the conjunctiva. Patients can also display jugular venous distention and facial edema. Associated injuries include pulmonary contusion, myocardial contusion, hemo/pneumothorax, and broken ribs.

A neonatal stroke is one that occurs in the first 28 days of life, though a late presentation is not uncommon (as contrasted with perinatal stroke, which occurs from 28 weeks gestation through the first 7 days of life). 80% of neonatal strokes are ischemic, and their presentation is varied, making diagnosis very difficult. The most common manifestation of neonatal strokes are seizures, but other manifestations include lethargy, hypotonia, apnoea, and hemiparesis. Seizures can be focal or generalized in nature. Stroke accounts for about 10% of seizures in term neonates.