The immediate effects include reducing the size of gas embolisms and raising the partial pressures of all gases present according to Henry's law.

[1][2][3] Hyperbaric oxygen therapy (HBOT), the medical use of greater than 99% oxygen at an ambient pressure higher than atmospheric pressure, and therapeutic recompression for decompression illness, intended to reduce the injurious effects of systemic gas bubbles by physically reducing their size and providing improved conditions for elimination of bubbles and excess dissolved gas.

The equipment required for hyperbaric oxygen treatment consists of a pressure vessel for human occupancy, which may be of rigid or flexible construction, and a means of a controlled atmosphere supply.

HBOT found early use in the treatment of decompression sickness, and has also shown great effectiveness in treating conditions such as gas gangrene and carbon monoxide poisoning.

[9] In the United States the Undersea and Hyperbaric Medical Society, known as UHMS, lists approvals for reimbursement for certain diagnoses in hospitals and clinics.

The use of HBOT may be justified to selected patients and tissues, but further research is required to establish the best people to treat and timing of any HBO therapy.

[66][67] Children receiving HBOT were reported to experience seizures and the need for tympanostomy tubes to equalize ear pressure, though the incidence was not clear.

[68] A 2012 review article in the journal, Targeted Oncology, reports that "there is no evidence indicating that HBO neither acts as a stimulator of tumor growth nor as an enhancer of recurrence.

"[69] Low-quality evidence suggests that hyperbaric oxygen therapy may reduce the pain associated with an acute migraine headache in some cases.

[70] Patients who are having extreme difficulty breathing – acute respiratory distress syndrome – are commonly given oxygen and there have been limited trials of hyperbaric equipment in such cases.

[77][page needed] Also, the treatment may raise the issue of occupational health and safety (OHS), for chamber inside attendants, who should not be compressed if they are unable to equalise ears and sinuses.

This is due to the high affinity of fetal hemoglobin for CO.[77][page needed] The therapeutic consequences of HBOT and recompression result from multiple effects.

[10][84] The increased overall pressure is of therapeutic value in the treatment of decompression sickness and air embolism as it provides a physical means of reducing the volume of inert gas bubbles within the body;[85] Exposure to this increased pressure is maintained for a period long enough to ensure that most of the bubble gas is dissolved back into the tissues, removed by perfusion and eliminated in the lungs.

[86][87] For many other conditions, the therapeutic principle of HBOT lies in its ability to drastically increase partial pressure of oxygen in the tissues of the body.

[84] A study suggests that exposure to hyperbaric oxygen (HBOT) might also mobilize stem/progenitor cells from the bone marrow by a nitric oxide-dependent mechanism.

[92][93][94] to portable, air inflated "soft" chambers that can operate at between 0.3 and 0.5 bars (4.4 and 7.3 psi) above atmospheric pressure with no supplemental oxygen, and longitudinal zipper closure.

[96] The geometries in general use include:[96] Low pressure, small diameter chambers may use large cylindrical windows fitted inside the metal structure.

For extremely serious cases resulting from very deep dives, the treatment may require a chamber capable of a maximum pressure of 8 bars (120 psi), the equivalent of 70 metres (230 ft) of water, and the ability to supply heliox as a breathing gas.

The Undersea and Hyperbaric Medical Society (UHMS) publishes a report that compiles the latest research findings and contains information regarding the recommended duration and pressure of the longer-term conditions.

The American Medical Association is opposed to home use or any other use of hyperbaric chambers if it is not "in facilities with appropriately trained staff including physician supervision and prescription and only when the intervention has scientific support or rationale" due demonstrated hazard [99] In the US, these "mild personal hyperbaric chambers" are categorized by the FDA as CLASS II medical devices and requires a prescription in order to purchase one or take treatments.

FDA approved oxygen concentrators for human consumption in confined areas used for HBOT are regularly monitored for purity (±1%) and flow (10 to 15 liters per minute outflow pressure).

The cause is not fully explained, but evidence suggests that lifetime exposure of the lens to high partial pressure oxygen may be a major factor.

Authorities have warned of potential risks to patients receiving treatment in unlicensed facilities, notably in Israel,[111] Canada,[112] and the United States.

[113] In Italy, the use of hyperbaric chambers for therapy was severely restricted to limited medical settings after a serious fire which killed ten patients in 1997.

In the U.S. state of North Carolina, several cities including Durham, Raleigh and Charlotte have ordered operators of mild hyperbaric oxygen therapy to close to protect public safety due to a risk of fire.

In the United Kingdom most chambers are financed by the National Health Service, although some, such as those run by Multiple Sclerosis Therapy Centres, are non-profit.

[129] Rats subjected to HBOT after some time following the acute phase of experimentally-induced stroke showed reduced inflammation, increased brain-derived neurotrophic factor, and evidence of neurogenesis.

[135] Orval J Cunningham, a professor of anesthesia at the University of Kansas in the early 1900s observed that people with circulatory disorders did better at sea level than at altitude and this formed the basis for his use of hyperbaric air.

Shortly after its discovery, there were reports of toxic effects of hyperbaric oxygen on the central nervous system and lungs, which delayed therapeutic applications until 1937, when Behnke and Shaw first used it in the treatment of decompression sickness.

[135] In 1955 and 1956 Churchill-Davidson, in the UK, used hyperbaric oxygen to enhance the radiosensitivity of tumours, while Ite Boerema [nl], at the University of Amsterdam, successfully used it in cardiac surgery.