Oxygen & Hyperbaric - The basics
HBOT basics: what pressure + oxygen actually do
Last updated 3 October 2026. How we write this .
Hyperbaric oxygen therapy means breathing pure oxygen in a chamber pressurised above normal air pressure. The pressure shrinks gas bubbles and forces far more oxygen to dissolve in your blood plasma.
Start here Hyperbaric oxygen therapy means breathing pure oxygen in a chamber pressurised above normal air pressure. The pressure shrinks gas bubbles and forces far more oxygen to dissolve in your blood plasma.
By the end, you will understand What does the pressure actually do? How much extra oxygen gets into your blood? What is HBOT proven to treat? Key words used in this lesson Half-life The time taken for the amount of a substance in the body to fall by half. Short half-lives are one reason many peptides behave very differently in a living body than in a test tube. Meta-analysis A statistical method for combining the results of multiple studies into a single estimate. More reliable than any one study, but bounded by the quality of what went into it. Hyperbaric Above normal atmospheric pressure. Hyperbaric oxygen therapy involves breathing oxygen in a pressurised chamber, which increases the amount dissolved in the blood. Full plain-English glossary
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Hyperbaric oxygen therapy, or HBOT, means breathing 100% oxygen inside a chamber where the air pressure is raised above normal. That combination does two physical things. It squeezes gas bubbles smaller, and it makes much more oxygen dissolve directly into the liquid part of your blood. Those two effects explain where HBOT clearly works, and why many claims made for it reach well beyond the evidence.
What does the pressure actually do?
Normal air pressure at sea level is about 100 kilopascals (kPa), or one atmosphere. NHS England defines HBOT as oxygen delivered at more than that, so that more of it dissolves in the blood plasma. Treatment pressures are usually a multiple of that: the UK HOT2 trial described below used 2.4 atmospheres, and the textbook oxygen figures that follow are for three.
Pressure changes the volume of gas. Boyle's law says a fixed amount of gas shrinks as the pressure on it rises, and at three atmospheres a bubble ends up at about a third of its size. That is the basis for treating decompression illness, where nitrogen bubbles form in the blood and tissues of divers who surface too fast, and gas embolism, where air gets into the circulation, often by accident during a medical procedure. Squeezing a blocking bubble moves it into smaller vessels, and breathing pure oxygen helps the nitrogen inside it dissolve away.
The same law explains the most common side effect. Air spaces in the body, such as the middle ear and the sinuses, get squeezed as pressure rises. If they cannot equalise, the result is pain or injury, which doctors call barotrauma.
How much extra oxygen gets into your blood?
Normally almost all the oxygen in your blood travels attached to haemoglobin inside red blood cells. Very little dissolves in the plasma. A 1998 BMJ review by Richard Leach, John Rees and Peter Wilmshurst puts numbers on the change. At sea level, plasma carries about 3 ml of oxygen per litre. At three atmospheres on pure oxygen, that approaches 60 ml per litre, which is almost enough to meet the resting needs of many tissues with no help from haemoglobin at all.
This is why HBOT is used when haemoglobin itself is out of action. Carbon monoxide binds to haemoglobin about 240 times more strongly than oxygen does, so the blood stops carrying oxygen properly. Leach and colleagues give the half-life of carbon monoxide in the blood as 240 to 320 minutes when breathing air, 80 to 100 minutes on pure oxygen at normal pressure, and about 20 minutes with hyperbaric oxygen.
What is HBOT proven to treat?
Fewer things than chamber marketing suggests, and the answer depends on who you ask. In England, the NHS routinely funds HBOT for just two conditions: decompression illness, around 150 to 200 cases a year, and gas embolism, around 10 a year. For decompression illness, NHS England describes it as the only treatment. It does not routinely fund HBOT for carbon monoxide poisoning, diabetic foot ulcers, severe soft tissue infections or tissue damage after pelvic radiotherapy.
The US-based Undersea and Hyperbaric Medical Society takes a broader view. Its list of accepted uses runs to 15 numbered categories, adding carbon monoxide poisoning, gas gangrene, crush injuries, selected problem wounds, delayed radiation injury, severe anaemia and sudden hearing loss, among others. None of the entries on either list is general wellness, anti-ageing or sports recovery.
Carbon monoxide shows why experts can disagree. In a 2002 trial in the New England Journal of Medicine, Lindell Weaver's team randomised 152 people with carbon monoxide poisoning. Six weeks later, 25% of the hyperbaric group had problems with memory and thinking, against 46% of those given oxygen at normal pressure. That is a large effect from a single trial, and NHS England still does not routinely commission HBOT for this use.
Where does the evidence get thinner?
Wounds are the classic example. A 2015 Cochrane review led by Peter Kranke pooled trials in diabetic foot ulcers. At six weeks, more ulcers had healed with HBOT, but the benefit was not evident at one year and there was no clear difference in major amputations. The reviewers said design flaws in the trials meant they were not confident in the results.
Sometimes a careful trial finds nothing at all. The UK's HOT2 trial, funded through Cancer Research UK and the NHS, tested HBOT in 84 people with long-term bowel problems after pelvic radiotherapy. It used a sham: the control group sat in a chamber at slightly raised pressure breathing ordinary air, so patients did not know which treatment they had. After 12 months there was no significant difference in bowel symptoms or bleeding. The authors wrote that this contrasted with the evidence used to justify existing practice.
That pattern is the fairest summary of where HBOT stands. The physics is solid, and the uses tied directly to it, bubbles and carbon monoxide, have the strongest case. The further a claim moves from that physics, towards chronic wounds, recovery or general wellbeing, the smaller and less consistent the supporting trials become.
Is HBOT safe?
In a properly run unit, serious harm is uncommon, but side effects are not. In HOT2, ear pain affected 28% of the HBOT group and 21% of the sham group, and changes in how the eyes focus affected 30% against 11%. A 2023 meta-analysis of 24 trials with 1,497 participants found adverse effects in 30% of people having HBOT, against 10% of controls. Ear discomfort was the most common, and problems were more frequent at pressures above 2.0 atmospheres.
Leach and colleagues list the rarer risks: seizures from oxygen toxicity, lung irritation with repeated sessions, and temporary short-sightedness that can last weeks or months. They name fire as the most common fatal complication, because a high concentration of oxygen raises the risk of fire. In August 2025 the US Food and Drug Administration wrote to health professionals after reports of HBOT chamber fires that caused serious injuries and deaths, urging them to follow manufacturers' safety instructions.
If you are offered HBOT for anything outside the recognised lists, it is reasonable to ask what condition it is meant to treat and what trial evidence supports that use. For any symptom that worries you, start with your GP or NHS 111.
This lesson is general education about how hyperbaric oxygen works, not medical advice. Decisions about hyperbaric treatment belong with an NHS or specialist hyperbaric doctor.
Related in this classroom The studies behind this page Hyperbaric oxygen therapy (ABC of oxygen) - BMJ, 1998Reviewing hyperbaric oxygen services: consultation guide - NHS England, 2024Hyperbaric oxygen therapy indications - Undersea and Hyperbaric Medical Society, 15th editionHyperbaric oxygen for acute carbon monoxide poisoning - New England Journal of Medicine, 2002Hyperbaric oxygen therapy for chronic wounds - Cochrane Database of Systematic Reviews, 2015Hyperbaric oxygen for patients with chronic bowel dysfunction after pelvic radiotherapy (HOT2): a randomised, double-blind, sham-controlled phase 3 trial - The Lancet Oncology, 2016Adverse effects of hyperbaric oxygen therapy: a systematic review and meta-analysis - Frontiers in Medicine, 2023Follow instructions for safe use of hyperbaric oxygen therapy devices: letter to health care providers - US Food and Drug Administration, 2025Sources for this classroom
Educational content only. Not medical advice, diagnosis or treatment. Always consult a qualified healthcare professional before changing your health regimen.