What Is Hyperbaric Oxygen Therapy? How HBOT Works and When It Helps
How HBOT works (Henry's law and dissolved oxygen), what a hyperbaric chamber does, how often sessions are given and which conditions it is proven to treat.
This content is for informational purposes only and does not constitute medical advice. Always consult a healthcare professional. Read full disclaimer
Hyperbaric oxygen therapy (HBOT) means breathing close to 100% oxygen inside a chamber pressurised above normal air pressure, usually 2 to 3 atmospheres absolute (ATA) for medical treatment. The pressure forces much more oxygen to dissolve into your blood plasma: at 3 ATA, plasma carries about 60 mL of oxygen per litre, roughly what resting tissues use, compared with about 3 mL per litre at normal pressure. HBOT is an established treatment for a defined list of conditions, such as decompression sickness, carbon monoxide poisoning and some non-healing wounds, but many popular wellness claims are unproven.
What Does a Hyperbaric Chamber Do?
A hyperbaric chamber raises the pressure around your whole body while you breathe oxygen. Pressure is measured in ATA, where 1 ATA is normal sea-level air pressure. According to StatPearls, pressure rises by 1 atmosphere for every 10 metres of seawater, so a treatment at 2.0 ATA exposes you to the same pressure as being 10 metres underwater, and 3.0 ATA to the pressure at about 20 metres.
The Undersea and Hyperbaric Medical Society (UHMS) defines clinical HBOT as treatment of the whole body in a hard-sided chamber at not less than 2.0 ATA, breathing medical-grade oxygen (over 99% pure), typically for 90 to 120 minutes per treatment, on a physician's prescription.
There are two main chamber designs:
- Monoplace chambers hold one person and are usually filled with pure oxygen.
- Multiplace chambers hold several patients (typically 2 to 14, according to a 2011 review by Thom). The chamber is pressurised with air and patients breathe oxygen through a tight-fitting mask or hood.
Our guide to chamber types compares hard-shell and soft-shell designs in more detail.
How Does HBOT Work?
HBOT works through basic gas laws plus biological effects that build up over repeated sessions.
1. More oxygen dissolves in plasma (Henry's law)
Normally, most of the oxygen in your blood is carried by haemoglobin inside red blood cells. Henry's law says the amount of gas that dissolves in a liquid is proportional to the pressure of that gas. Breathing pure oxygen at high pressure therefore dissolves far more oxygen directly into plasma.
StatPearls gives the numbers: resting tissues need about 60 mL of oxygen per litre of blood. At 1 ATA, plasma holds about 3 mL per litre. At 3 ATA with HBOT, this rises to about 60 mL per litre, on top of the oxygen carried by haemoglobin. Oxygen can then diffuse roughly four times further into tissue than normal. Thom's review reports that arterial oxygen tension often exceeds 2,000 mmHg during treatment, with 200 to 400 mmHg reaching tissues.
The classic demonstration was "Life Without Blood", published in 1959 by Ite Boerema, who kept pigs alive under hyperbaric oxygen after draining their red blood cells.
2. Pressure shrinks gas bubbles (Boyle's law)
Boyle's law says the volume of a gas shrinks as pressure rises. In decompression sickness and gas embolism, where bubbles block blood vessels, pressure makes the bubbles smaller and helps the gas go back into solution, while breathing oxygen speeds removal of nitrogen.
3. Effects that build over a course
Repeated high-oxygen exposures also trigger biological responses. Thom's review describes stimulation of new blood vessel growth, including mobilisation of stem and progenitor cells from bone marrow, and a temporary reduction in the ability of white blood cells to stick to damaged vessels after exposure at 2.8 to 3.0 ATA. These effects help explain why wound and radiation injury protocols involve many sessions rather than one.
What Happens During a Session?
- Screening. A doctor checks for conditions that make HBOT unsafe, such as an untreated collapsed lung, and reviews your medicines.
- Preparation. You change into cotton or other approved clothing and remove make-up, oils and deodorant, because pressurised oxygen is a fire hazard.
- Compression. As the pressure rises, your ears feel full, much like during a plane's descent. You equalise by swallowing or yawning, and the operator can slow down if your ears do not clear.
- Treatment. You breathe oxygen at the prescribed pressure, usually with short air breaks, which reduce the risk of oxygen toxicity. Medical treatments typically last 90 to 120 minutes.
- Decompression. Pressure is brought back down slowly before the chamber is opened.
For a step-by-step walkthrough, read what to expect at your first HBOT session. The most common side effects are ear pressure and temporary short-sightedness; see HBOT side effects and safety.
How Often Should You Do Hyperbaric Oxygen Therapy?
It depends entirely on the condition. UHMS notes that most disorders need a series of treatments delivered daily for several weeks. Thom's review describes sessions of about 1.5 to 2 hours, given one to three times a day depending on the indication, and wound protocols of daily treatment for 20 to 40 days.
| Condition | Typical course described in the evidence |
|---|---|
| Sudden sensorineural hearing loss | 2.0 to 2.5 ATA for 90 minutes daily, 10 to 20 treatments (UHMS), ideally starting within two weeks |
| Diabetic foot ulcer | 30 to 40 sessions, five days a week, at 2.4 to 2.5 ATA in major trials; reviewed every 30 days under Medicare rules |
| Chronic refractory osteomyelitis | Once a day for 90 to 120 minutes at 2 to 3 ATA, usually 20 to 40 sessions (StatPearls) |
| Jaw osteoradionecrosis | The Marx protocol described by UHMS: 30 daily sessions, followed by 10 more after surgery |
| Reperfusion injuries (damage when blood flow returns to starved tissue) | A few days rather than weeks, at about 2.5 to 3.0 ATA, sometimes several times a day (Thom) |
There is no evidence-based schedule for wellness use. UHMS describes "mild" hyperbaric sessions below 1.5 ATA as unproven, so any frequency recommended for general wellness is a clinic's choice rather than a medical standard.
Do Hyperbaric Chambers Work?
For the right conditions, yes. For many others, the evidence is weak or negative.
Accepted indications. The UHMS indication list covers air or gas embolism, carbon monoxide poisoning, gas gangrene, crush injury, decompression sickness, central retinal artery occlusion, selected problem wounds, severe anaemia, intracranial abscess, necrotising soft-tissue infections, refractory osteomyelitis, delayed radiation injury, compromised grafts and flaps, acute thermal burns, sudden sensorineural hearing loss and avascular necrosis. The European Committee for Hyperbaric Medicine (ECHM) and Medicare publish similar lists. Our guide to accepted HBOT indications compares them side by side.
Even accepted uses vary in strength. For diabetic foot ulcers, the Cochrane review found faster healing at six weeks but no clear benefit at one year, and randomised trials disagree.
Not proven. The FDA's consumer guidance stated that HBOT devices are not proven to cure cancer, Lyme disease, autism or Alzheimer's disease, and that it had not cleared chambers for conditions beyond its list of 13. The ECHM went further in 2016 and recommended against HBOT for autism spectrum disorders, multiple sclerosis, cerebral palsy, tinnitus, placental insufficiency and the acute phase of stroke. UHMS considers treatment for traumatic brain injury investigational.
Medical HBOT vs Mild Hyperbaric Chambers
| Factor | Medical HBOT | "Mild" hyperbaric sessions |
|---|---|---|
| Chamber | Hard-sided chamber built to pressure vessel and fire codes | Often soft, zippered chambers; FDA cleared these only for altitude sickness |
| Pressure | At least 2.0 ATA; evidence-based treatments usually 1.9 to 3.0 ATA | Below 1.5 ATA (UHMS definition) |
| Oxygen | Medical-grade oxygen, over 99% pure | Often well below 95%, through masks that do not seal |
| Supervision | Physician prescription and oversight | Generally not prescribed or supervised by a physician, per UHMS |
| Evidence | Accepted for a defined list of conditions | Unproven, per UHMS |
Emergencies: Do Not Wait for a Clinic
Decompression sickness, gas embolism and carbon monoxide poisoning are emergencies. Call your local emergency number first. Divers can then contact the Divers Alert Network (DAN) emergency hotline on +1-919-684-9111, staffed 24 hours a day. Emergency HBOT is arranged through emergency services and hospital chambers, not booked at a wellness centre.
Finding a Hyperbaric Clinic
As of September 2026, hyperbaric.app lists 1,632 active facilities in 68 countries; 1,170 are listed with hard-shell chambers and 149 with soft-shell chambers. You can search for hyperbaric clinics near you or explore the directory statistics.
Sources
- Jones MW, Brett K, Han N, Cooper JS, Wyatt HA. Hyperbaric physics. StatPearls. Updated 31 January 2024.
- Thom SR. Hyperbaric oxygen: its mechanisms and efficacy. Plastic and Reconstructive Surgery. 2011;127 Suppl 1:131S-141S.
- Undersea and Hyperbaric Medical Society, Indications for hyperbaric oxygen therapy
- Mathieu D, Marroni A, Kot J. Tenth European Consensus Conference on Hyperbaric Medicine: recommendations for accepted and non-accepted clinical indications and practice of hyperbaric oxygen treatment. Diving and Hyperbaric Medicine. 2017;47(1):24-32.
- Kranke P, Bennett MH, Martyn-St James M, Schnabel A, Debus SE, Weibel S. Hyperbaric oxygen therapy for chronic wounds. Cochrane Database of Systematic Reviews. 2015;(6):CD004123.
- Löndahl M, Katzman P, Nilsson A, Hammarlund C. Hyperbaric oxygen therapy facilitates healing of chronic foot ulcers in patients with diabetes. Diabetes Care. 2010;33(5):998-1003.
- Cooper JS, Hanley ME, Hendriksen S. Hyperbaric treatment of chronic refractory osteomyelitis. StatPearls. Updated 2 June 2025.
- Gawdi R, Yrastorza J, Cooper JS. Hyperbaric oxygen therapy contraindications. StatPearls. Updated 27 May 2025.
- Centers for Medicare & Medicaid Services, National Coverage Determination 20.29: Hyperbaric Oxygen Therapy
- U.S. Food and Drug Administration, Hyperbaric oxygen therapy: get the facts (consumer update, content current as of 26 July 2021; archived copy)
- U.S. Food and Drug Administration, Follow instructions for safe use of hyperbaric oxygen therapy devices: letter to health care providers (25 August 2025)
- Divers Alert Network, Emergency assistance
This article is for information only and is not medical advice. Whether HBOT is appropriate for you should be decided with a qualified physician.