Recompression treatment in a hyperbaric chamber was initially used as a life-saving tool to treat decompression sickness in caisson workers and divers who stayed too long at depth and developed decompression sickness. Now, it is a highly specialized treatment modality that has been found to be effective in the treatment of many conditions where the administration of oxygen under pressure has been found to be beneficial. Studies have shown it to be quite effective in some 13 indications approved by the Undersea and Hyperbaric Medical Society.

Hyperbaric oxygen treatment is generally preferred when effective, as it is usually a more efficient and lower risk method of reducing symptoms of decompression illness, However, in some cases recompression to pressures where oxygen toxicity is unacceptable may be required to eliminate the bubbles in the tissues that cause the symptoms.

The administration of oxygen as a medical intervention is common in diving medicine, both for first aid and for longer term treatment.

Immediate treatment with 100% oxygen, followed by recompression in a hyperbaric chamber, will in most cases result in no long-term effects. However, permanent long-term injury from DCS is possible. Three-month follow-ups on diving accidents reported to DAN in 1987 showed 14.3% of the 268 divers surveyed had ongoing symptoms of Type II DCS, and 7% from Type I DCS. Long-term follow-ups showed similar results, with 16% having permanent neurological sequelae.

The most straightforward way to avoid nitrogen narcosis is for a diver to limit the depth of dives. Since narcosis becomes more severe as depth increases, a diver keeping to shallower depths can avoid serious narcosis. Most recreational dive schools will only certify basic divers to depths of , and at these depths narcosis does not present a significant risk. Further training is normally required for certification up to on air, and this training should include a discussion of narcosis, its effects, and cure. Some diver training agencies offer specialized training to prepare recreational divers to go to depths of , often consisting of further theory and some practice in deep dives under close supervision. Scuba organizations that train for diving beyond recreational depths, may forbid diving with gases that cause too much narcosis at depth in the average diver, and strongly encourage the use of other breathing gas mixes containing helium in place of some or all of the nitrogen in air – such as trimix and heliox – because helium has no narcotic effect. The use of these gases forms part of technical diving and requires further training and certification.

While the individual diver cannot predict exactly at what depth the onset of narcosis will occur on a given day, the first symptoms of narcosis for any given diver are often more predictable and personal. For example, one diver may have trouble with eye focus (close accommodation for middle-aged divers), another may experience feelings of euphoria, and another feelings of claustrophobia. Some divers report that they have hearing changes, and that the sound their exhaled bubbles make becomes different. Specialist training may help divers to identify these personal onset signs, which may then be used as a signal to ascend to avoid the narcosis, although severe narcosis may interfere with the judgement necessary to take preventive action.

Deep dives should be made only after a gradual training to test the individual diver's sensitivity to increasing depths, with careful supervision and logging of reactions. Diving organizations such as Global Underwater Explorers (GUE) emphasize that such sessions are for the purpose of gaining experience in recognizing the onset symptoms of narcosis for an individual , which are somewhat more repeatable than for the average group of divers. Scientific evidence does not show that a diver can train to overcome any measure of narcosis at a given depth or become tolerant of it.

Equivalent narcotic depth (END) is a commonly used way of expressing the narcotic effect of different breathing gases. The National Oceanic and Atmospheric Administration (NOAA) Diving Manual now states that oxygen and nitrogen should be considered equally narcotic. Standard tables, based on relative lipid solubilities, list conversion factors for narcotic effect of other gases. For example, hydrogen at a given pressure has a narcotic effect equivalent to nitrogen at 0.55 times that pressure, so in principle it should be usable at more than twice the depth. Argon, however, has 2.33 times the narcotic effect of nitrogen, and is a poor choice as a breathing gas for diving (it is used as a drysuit inflation gas, owing to its low thermal conductivity). Some gases have other dangerous effects when breathed at pressure; for example, high-pressure oxygen can lead to oxygen toxicity. Although helium is the least intoxicating of the breathing gases, at greater depths it can cause high pressure nervous syndrome, a still mysterious but apparently unrelated phenomenon. Inert gas narcosis is only one factor influencing the choice of gas mixture; the risks of decompression sickness and oxygen toxicity, cost, and other factors are also important.

Because of similar and additive effects, divers should avoid sedating medications and drugs, such as marijuana and alcohol before any dive. A hangover, combined with the reduced physical capacity that goes with it, makes nitrogen narcosis more likely. Experts recommend total abstinence from alcohol for at least 12 hours before diving, and longer for other drugs. Abstinence time needed for marijuana is unknown, but owing to the much longer half-life of the active agent of this drug in the body, it is likely to be longer than for alcohol.

Professional divers are screened for risk factors during initial and periodical medical examination for fitness to dive. In most cases recreational divers are not medically screened, but are required to provide a medical statement before acceptance for training in which the most common and easy to identify risk factors must be declared. If these factors are declared, the diver may be required to be examined by a medical practitioner, and may be disqualified from diving if the conditions indicate.

Asthma, Marfan syndrome, and COPD pose a very high risk of pneumothorax. In some countries these may be considered absolute contraindications, while in others the severity may be taken into consideration. Asthmatics with a mild and well controlled condition may be permitted to dive under restricted circumstances.

Decompression sickness should be suspected if any of the symptoms associated with the condition occurs following a drop in pressure, in particular, within 24 hours of diving. In 1995, 95% of all cases reported to Divers Alert Network had shown symptoms within 24 hours. An alternative diagnosis should be suspected if severe symptoms begin more than six hours following decompression without an altitude exposure or if any symptom occurs more than 24 hours after surfacing. The diagnosis is confirmed if the symptoms are relieved by recompression. Although MRI or CT can frequently identify bubbles in DCS, they are not as good at determining the diagnosis as a proper history of the event and description of the symptoms.

First aid is common for both DCS and AGE:

- Monitor the patient for responsiveness, airway, breathing and circulation, resuscitate if necessary.

- Treat for shock.

- Lay the patient on their back, or for drowsy, unconscious, or nauseated victims, on their side.

- Administer 100% oxygen as soon as possible.

- Seek immediate medical assistance, locate a hospital with hyperbaric facilities and plan for possible transport.

- Allow the patient to drink water or isotonic fluids only if responsive, stable, and not suffering from nausea or stomach pain. Administration of intravenous saline solution is preferable.

- Record details of recent dives and responses to first aid treatment and provide to the treating medical specialist. The diving details should include depth and time profiles, breathing gases used and surface intervals.

Narcosis is potentially one of the most dangerous conditions to affect the scuba diver below about . Except for occasional amnesia of events at depth, the effects of narcosis are entirely removed on ascent and therefore pose no problem in themselves, even for repeated, chronic or acute exposure. Nevertheless, the severity of narcosis is unpredictable and it can be fatal while diving, as the result of illogical behavior in a dangerous environment.

Tests have shown that all divers are affected by nitrogen narcosis, though some experience lesser effects than others. Even though it is possible that some divers can manage better than others because of learning to cope with the subjective impairment, the underlying behavioral effects remain. These effects are particularly dangerous because a diver may feel they are not experiencing narcosis, yet still be affected by it.

Treatment for the "Decompression Sickness" and the "Arterial Gas Embolism" components of DCI may differ significantly. Refer to the separate treatments under those articles.

A significant part of entry level diver training is focused on understanding the risks and procedural avoidance of barotrauma. Professional divers and recreational divers with rescue training are trained in the basic skills of recognizing and first aid management of diving barotrauma.

Dysbarism refers to medical conditions resulting from changes in ambient pressure. Various activities are associated with pressure changes. underwater diving is the most frequently cited example, but pressure changes also affect people who work in other pressurized environments (for example, caisson workers), and people who move between different altitudes.

As a general rule, any diver who has breathed gas under pressure at any depth who surfaces unconscious, loses consciousness soon after surfacing, or displays neurological symptoms within about 10 minutes of surfacing should be assumed to be suffering from arterial gas embolism.

Symptoms of arterial gas embolism may be present but masked by environmental effects such as hypothermia, or pain from other obvious causes. Neurological examination is recommended when there is suspicion of lung overexpansion injury. Symptoms of decompression sickness may be very similar to, and confused with, symptoms of arterial gas embolism, however, treatment is basically the same. Discrimination between gas embolism and decompression sickness may be difficult for injured divers, and both may occur simultaneously. Dive history may eliminate decompression sickness in many cases, and the presence of symptoms of other lung overexpansion injury would raise the probability of gas embolism.

It is likely that HPNS cannot be entirely prevented but there are effective methods to delay or change the development of the symptoms.

Utilizing slow rates of compression or adding stops to the compression have been found to prevent large initial decrements in performance.

Including other gases in the helium–oxygen mixture, such as nitrogen (creating trimix) or hydrogen (producing hydreliox) suppresses the neurological effects.

Alcohol, anesthetics and anticonvulsant drugs have had varying results in suppressing HPNS with animals. None are currently in use for humans.

If a patent foramen ovale (PFO) is suspected, an examination by echocardiography may be performed to diagnose the defect. In this test, very fine bubbles are introduced into a patient's vein by agitating saline in a syringe to produce the bubbles, then injecting them into an arm vein. A few seconds later, these bubbles may be clearly seen in the ultrasound image, as they travel through the patient's right atrium and ventricle. At this time, bubbles may be observed directly crossing a septal defect, or else a patent foramen ovale may be opened temporarily by asking the patient to perform the Valsalva maneuver while the bubbles are crossing through the right heart – an action which will open the foramen flap and show bubbles passing into the left heart. Such bubbles are too small to cause harm in the test, but such a diagnosis may alert the patient to possible problems which may occur from larger bubbles, formed during activities like underwater diving, where bubbles may grow during decompression. A PFO test may be recommended for divers intending to expose themselves to relatively high decompression stress in deep technical diving.

Management has generally been reported to be conservative, though deaths have been reported.

- Removal from water

- Observation

- Diuretics and / or Oxygen when necessary

- Episodes are generally self-limiting in the absence of other medical problems

SIPE is estimated to occur in 1-2% of competitive open-water swimmers, with 1.4% of triathletes, 1.8% of combat swimmers and 1.1% of divers and swimmers reported in the literature.

If the diver has not been exposed to excessive depth and decompression and presents as DON, there may be a predisposition for the condition. Diving should be restricted to shallow depths. Divers who have suffered from DON are at increased risk of future fracture of a juxta-articular lesion during a dive, and may face complications with future joint replacements. Because of the young age of the population normally affected, little data is available regarding joint replacement complications.

There is the potential for worsening of DON for any diving where there might be a need for decompression, experimental or helium diving. Physically stressful diving should probably be restricted, both in sport diving and work diving due to the possibility of unnecessary stress to the joint. Any diving should be less than 40 feet/12 meters. These risks are affected by the degree of disability and by the type of lesion (juxta-articular or shaft).

Arterial gas embolism (AGE) is a complication of lung barotrauma of ascent. It occurs when breathing gas is introduced to the circulation on the arterial side via lung over-pressure trauma. AGE can present in similar ways to arterial blockages seen in other medical situations. Affected people may suffer strokes, with paralysis or numbness down one side; they may suffer heart attacks; they may suffer pulmonary embolism with shortness of breath and chest pain. It is often impossible to distinguish AGE from DCS, but luckily it is rarely necessary for physicians to be able to distinguish between the two, as treatment is the same. Sometimes AGE and DCS are lumped into a single entity, Decompression Illness (DCI).

The hypercapnic state is routinely used to calibrate blood-oxygen-level dependent functional magnetic resonance imaging (BOLD fMRI), a modality that is sensitive to changes in blood oxygenation. However, this calibration crucially relies on the assumption that hypercapnia has no effect on neuronal function, which is a matter of debate.

Prevention is a more successful strategy than treatment. By using the most conservative decompression schedule reasonably practicable, and by minimizing the number of major decompression exposures, the risk of DON may be reduced. Prompt treatment of any symptoms of decompression sickness (DCS) with recompression and hyperbaric oxygen also reduce the risk of subsequent DON.

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.

HPNS has two components, one resulting from the speed of compression and the other from the absolute pressure. The compression effects may occur when descending below at rates greater than a few metres per minute, but reduce within a few hours once the pressure has stabilised. The effects from depth become significant at depths exceeding and remain regardless of the time spent at that depth.

The susceptibility of divers and animals to HPNS varies over a wide range depending on the individual, but has little variation between different dives by the same diver.

Hypercapnia is generally defined as a blood gas carbon dioxide level over 45 mmHg. Since carbon dioxide is in equilibrium with carbonic acid in the blood, hypercapnia can drive serum pH down, resulting in a respiratory acidosis. Clinically, the effect of hypercapnia on pH is estimated using the ratio of the arterial pressure of carbon dioxide to the concentration of bicarbonate ion, PCO/[HCO].

Administration of oxygen at 15 litres per minute by face mask or bag valve mask is often sufficient, but tracheal intubation with mechanical ventilation may be necessary. Suctioning of pulmonary oedema fluid should be balanced against the need for oxygenation. The target of ventilation is to achieve 92% to 96% arterial saturation and adequate chest rise. Positive end-expiratory pressure will generally improve oxygenation. Drug administration via peripheral veins is preferred over endotracheal administration. Hypotension remaining after oxygenation may be treated by rapid crystalloid infusion. Cardiac arrest in drowning usually presents as asystole or pulseless electrical activity. Ventricular fibrillation is more likely to be associated with complications of pre-existing coronary artery disease, severe hypothermia, or the use of epinephrine or norepinephrine.

The Fédération dentaire internationale describes 4 classes of barodontalgia. The classes are based on signs and symptoms. They also provide specific and valuable recommendations for therapeutic intervention.