What does a hyperbaric chamber do? Pressure, oxygen and chamber types explained

A hyperbaric chamber does two things at once: it raises the pressure around your body, and it lets you breathe concentrated oxygen at that pressure. Together they increase the oxygen dissolved in your blood plasma well beyond what breathing normal air can achieve. That is the whole principle. Everything else about a chamber, from its size to whether it is steel or fabric, decides how much pressure and oxygen it can deliver, and so which research applies to it.

What does a hyperbaric chamber actually do?

NHS England describes hyperbaric oxygen therapy (HBOT) as the delivery of oxygen at a pressure greater than normal, so that a higher level of oxygen can be dissolved in the patient's blood plasma.

Most of the oxygen in your blood travels bound to haemoglobin inside red blood cells. In a healthy person that carrier is already close to full, so breathing more oxygen at normal pressure adds relatively little. Pressure works on a different part of the blood: the plasma, the liquid the cells travel in.

Henry's law says the amount of a gas that dissolves in a liquid is directly proportional to the pressure of that gas above it. A StatPearls review of hyperbaric physics gives the scale. Plasma holds roughly 3 mL of oxygen per litre at normal pressure. During HBOT at 3 ATA, that rises to about 60 mL per litre, which the review notes is roughly what resting tissue needs. That dissolved oxygen does not depend on red blood cells to deliver it.

The same review explains Boyle's law: at a constant temperature, the volume of a gas falls as pressure rises. That is why pressure itself treats gas bubbles in the blood after a diving injury or gas embolism, the two uses NHS England routinely commissions. It is also why your ears feel full as a chamber pressurises. Air in the middle ear compresses until you equalise it.

Mechanism is not the same as outcome. A plausible effect on oxygen justifies research. Only trials can show whether a particular person, with a particular problem, benefits.

What does ATA mean?

ATA stands for atmospheres absolute. It is the total pressure on the body, including the atmosphere above you. At sea level you are at about 1 ATA. StatPearls explains that seawater adds 1 atmosphere for every 10 metres of depth, so a diver 10 metres down is at 2 ATA, double the pressure at the surface.

Different bodies draw the line for clinical HBOT in different places, and the numbers matter:

• The Undersea and Hyperbaric Medical Society (UHMS) defines hyperbaric oxygen on its indications page as treatment in a hard-sided chamber at not less than 2.0 ATA while breathing medical-grade oxygen, with scientifically supported treatments usually between 1.9 and 3.0 ATA.
• The same page describes exposures below 1.5 ATA, often called mild hyperbaric oxygen, as unproven.
• The UHMS position statement on low-pressure fabric chambers draws a lower threshold, defining HBOT as breathing near 100 percent oxygen at 1.4 ATA or above, and states that all UHMS-approved indications currently require at least 2 ATA.

So when you read a study, look for three numbers: the pressure, the oxygen breathed, and how many sessions were given. A result at one pressure cannot automatically be moved to another.

Soft vs hard hyperbaric chambers: what is the difference?

Hard-sided monoplace chambers. A rigid pressure vessel for one person. A horizon scan published by the US Agency for Healthcare Research and Quality explains that only one patient is treated in a monoplace chamber, and that patients having HBOT typically breathe 100 percent oxygen at about 2 to 2.5 ATA. UHMS expects clinical chambers to meet recognised pressure vessel and fire-safety codes.

Hard-sided multiplace chambers. A pressurised room that can hold several patients and medical staff at once. A StatPearls review of patient selection notes that when a multiplace chamber is available there are usually no limitations, because a practitioner or attendant is with the patient during treatment, while some monoplace chambers lack features such as air breaks or monitoring ports.

Soft-shell chambers. Inflatable chambers made from fabric. The UHMS position statement describes low-pressure fabric chambers operating below 1.4 ATA, designed to be pressurised with air and cleared in the US only for acute mountain sickness. It states that mild hyperbaric exposure with air delivers no more oxygen to the body than breathing oxygen by mask at sea level. A 2026 narrative review describes mild hyperbaric oxygen as roughly 1.1 to 1.49 ATA, using air or oxygen from a concentrator, and concludes that its evidence is very limited, of poor quality, and often wrongly extrapolated from higher-pressure studies.

Chamber type	Construction	Occupants	Pressure described in the sources
Monoplace	Rigid pressure vessel	One person	Typically about 2 to 2.5 ATA in clinical HBOT
Multiplace	Rigid pressurised room	Several patients plus staff	Clinical HBOT pressures, usually 1.9 to 3.0 ATA
Low-pressure soft chamber	Inflatable fabric	One person	Below 1.4 ATA on air, per UHMS
Mild hyperbaric (general)	Usually soft chambers	One person	About 1.1 to 1.49 ATA, per a 2026 review

The shell material is not the point in itself. What matters is the pressure the chamber reaches, what you breathe inside it, and whether that combination matches the research you are reading.

What does chamber type mean for evidence and safety?

For evidence, almost all established hyperbaric indications come from rigid chambers at 2.0 ATA or higher. Results from those protocols describe those protocols. The main controlled trial at lower pressure is a 2025 double-blind study in which 47 adults with persistent symptoms after brain injury were analysed after 40 sessions at 1.5 ATA or sham; symptoms improved more with HBOT, but the trial recruited about a third of its intended sample and UHMS still regards this use as investigational.

For safety, the physics applies whatever the chamber is made of. Ear and sinus pressure happen during any pressure change. An untreated collapsed lung is dangerous in any hyperbaric chamber. And any oxygen-enriched space carries a fire risk, which is why UHMS warns that facilities without proper fire safety and construction standards put people at risk of serious injury. Lower pressure reduces some oxygen-related risks, but it does not remove the need for screening. Our guide to who HBOT is not suitable for covers the contraindications.

Where our chamber sits. HBOT Medics uses a single-person soft-shell chamber operating at 1.5 ATA. That is above the low-pressure fabric chambers the UHMS statement describes, and below the 2.0 ATA rigid-chamber protocols behind most accepted indications. We cannot present results from those higher-pressure protocols as what happens in our chamber, and our chamber page sets out how we compare research with our equipment.

What happens inside the chamber during a session?

At HBOT Medics one person uses the chamber at a time. Pressure rises gradually, and the operator explains how to clear your ears before you start. At pressure you breathe through the supplied oxygen system, you can communicate with the operator throughout, and a session can be stopped if needed. Pressure comes down gradually at the end.

For the full sequence from arrival to leaving, read what happens during treatment or our step-by-step session article. If you want to know how many sessions research protocols use, see how often you should have HBOT. When you are ready, you can see available times.