Mild HBOT vs clinical HBOT: what the ATA number actually means
If you have started looking into hyperbaric oxygen therapy in Australia, you have probably noticed that nobody agrees on what it is. One place has an inflatable bag in a converted garage for $80. Another has a steel chamber, a doctor and a $250 price tag. Both call it hyperbaric oxygen therapy.
Both are, technically, telling the truth.
The number that separates them is ATA, and almost nobody explains it. So here it is, properly, with the research attached.
What ATA means
ATA stands for atmospheres absolute. It is a measure of pressure.
At sea level, the column of air above you presses down at 1 ATA. That is your baseline right now. Ten metres under water is 2 ATA, because ten metres of seawater weighs about the same as the entire atmosphere above it. Twenty metres is 3 ATA.
A hyperbaric chamber does the same thing as descending in water, without the water. It raises the pressure around you above 1 ATA. A chamber running at 2.0 ATA is putting you under the same pressure you would feel at ten metres depth.
That is the whole device. Everything else is engineering.
Why raising the pressure does anything at all
Your blood carries oxygen two ways.
The first way is haemoglobin, the protein in your red blood cells. This does almost all the work. Breathing normal air at sea level, your haemoglobin is somewhere around 97 to 98 percent saturated. It is nearly full. You cannot meaningfully increase it, which is why breathing pure oxygen at normal pressure does far less than people assume.
The second way is dissolved oxygen, sitting free in the plasma, the watery part of your blood. Normally this is a rounding error, a tiny fraction of the total. But it obeys a different rule. The amount of gas that dissolves in a liquid is directly proportional to the pressure of that gas above the liquid. That is Henry’s Law, and it is the entire reason hyperbaric chambers exist.
Raise the pressure, raise the oxygen concentration, and the dissolved fraction climbs in a straight line while haemoglobin sits there already full. Push it far enough and you can carry enough dissolved oxygen in plasma alone to meet resting tissue demand.
Dissolved oxygen matters because it is not stuck inside red blood cells. Red blood cells are large and they need a functioning capillary to travel down. Dissolved oxygen is in the fluid, and fluid gets into places cells struggle to reach. That is why hospitals use hyperbaric oxygen for crush injuries, radiation-damaged tissue and non-healing wounds, where the problem is precisely that blood is not getting where it needs to go [1].I lso
So the mechanism is not mystical. It is a gas law. Which is also why the pressure number is not a marketing detail. It is the dose.
The 1.4 ATA line
Here is the thing the industry does not advertise.
The Undersea and Hyperbaric Medical Society, the professional body for the field, defines hyperbaric oxygen therapy as breathing near-100% oxygen inside a chamber pressurised to at least 1.4 ATA[2]. Below that, by the definition of the people who do this for a living, it is not hyperbaric oxygen therapy. The same definition explicitly excludes breathing 100% oxygen at normal pressure, and excludes applying oxygen to the skin.
Now hold that number next to the Australian wellness market. A large share of what is sold here as “mild hyperbaric” or “mHBOT” runs in a soft inflatable chamber at 1.3 to 1.35 ATA. That sits below the professional threshold.
It gets more awkward.
1.3 ATA is the placebo
When researchers run a clinical trial of hyperbaric oxygen, they need a control group. The control cannot just sit in a waiting room, because getting sealed into a chamber for an hour is an experience, and experiences have effects. So the control group is put in the chamber too, at a pressure low enough to be inert but high enough that they feel their ears pop and believe they are being treated.
That sham pressure is, conventionally, 1.3 ATA on air.
Mitchell and Bennett, two of the most published researchers in hyperbaric medicine, have argued the point directly: 1.3 ATA breathing air is not an active treatment [3]. It is the thing you compare the treatment against.
This creates an uncomfortable situation for a chunk of the wellness hyperbaric industry. A number of soft-chamber operators are selling, as the product, roughly what the research community uses as the placebo.
Two honest caveats, because this is the point where it would be easy to overstate.
First, 1.3 ATA on air is not identical to 1.3 ATA with high-concentration oxygen through a mask.
Adding concentrated oxygen at low pressure is a genuinely different exposure to breathing ordinary air at low pressure. It is a fair objection. It is also much less studied, so if you are being sold a 1.3 ATA oxygen protocol, be aware that you are in territory where good evidence is thin rather than territory where good evidence says it works.
Second, sham groups in these trials frequently improve.
In the post-concussion trials in particular, the sham arms got better at nearly the same rate as the treatment arms [4][5]. That is usually read as a placebo and participation effect. But you could also read it, as some researchers do, as evidence that low pressure does something. Nobody has resolved this, and anyone who tells you it is settled in either direction is overselling.
Where the results actually are
Look at where the positive research clusters and a pattern appears immediately.
Cognitive function in older adults. A randomised controlled trial of 63 healthy adults aged 64 and over ran three months of hyperbaric oxygen and found improved global cognitive function against controls, with the largest effects in attention and processing speed, alongside increased cerebral blood flow [6]. Pressure: 2.0 ATA.
Telomeres and cellular ageing. A prospective trial of 30 adults over 64 reported increased telomere length in isolated blood cells and a reduction in senescent T-helper cells after 60 sessions [7]. Pressure: 2.0 ATA.
Fibromyalgia. A crossover trial of 60 women found significant improvement in symptoms and quality of life, with changes visible on brain imaging [8]. Pressure: 2.0 ATA.
Post-COVID cognitive symptoms. A sham-controlled randomised trial of 73 people found improvements in global cognitive function, attention and executive function, with corresponding changes in brain perfusion [9]. Pressure: 2.0 ATA.
Four different research groups, four different problems, one pressure.
There is also direct evidence that pressure is dose. In a study of consecutive patients, sessions at 2.5 ATA mobilised roughly two to three times more circulating stem cells than sessions at 2.0 ATA [10]. That is a dose-response curve, measured in humans. Extrapolate it downward and 1.3 ATA looks like a very small dose indeed.
Compare that with the trials actually conducted at mild pressure. One randomised crossover study ran a single 60-minute exposure at 1.3 ATA on 100% oxygen in 14 participants. It lowered resting heart rate and shifted heart rate variability, but did not improve aerobic endurance [11]. A three-arm randomised trial compared 1.5 ATA, 2.0 ATA and sham in post-concussion syndrome and found no significant differences between any of them [12].
That is close to the entire mild-pressure randomised evidence base. It is not much, and it is not encouraging.
Hard shell versus soft shell
The pressure difference is not arbitrary. It is physics.
A soft-shell chamber is a sealed fabric envelope, usually urethane over a frame, inflated by a compressor. It is portable, relatively cheap, and it flexes. The material and the seams set a hard ceiling on the pressure differential it can safely hold, which in practice is around 1.3 to 1.4 ATA. Most soft chambers are pressurised with ambient air, with concentrated oxygen fed in through a mask or concentrator on top.
A hard-shell chamber is a rigid pressure vessel, typically steel or acrylic, engineered to hold a much larger differential. Clinical units run to 2.4 ATA and beyond. Hard-shell wellness chambers commonly run to 2.0. Because the vessel does not deform, the pressure you set is the pressure you get, and it is the same on session one and session fifty.
There is a comfort difference too, but it is secondary. The thing that matters is that soft shell means low pressure, structurally and unavoidably, and low pressure means low dose.
So what is clinical HBOT?
At the far end sits the hospital version. In Australia that means an accredited hyperbaric medicine unit, usually attached to a major hospital, running multi-place chambers at around 2.4 ATA on 100% oxygen with a doctor present and a nurse or technologist inside.
Those units treat a defined list of conditions with real evidence behind them. Decompression illness. Carbon monoxide poisoning. Non-healing diabetic foot ulcers, where a Cochrane review of 12 trials found significantly improved healing in the short term [13]. Late radiation tissue injury, supported by a 2023 Cochrane review of 18 studies [14]. Sudden sensorineural hearing loss, where a Cochrane review found significant hearing improvement [15]. The European Consensus Conference on Hyperbaric Medicine maintains the graded indication list [16].
That is a medical service for medical problems, delivered under medical supervision, and mostly funded accordingly. It is not what a wellness studio does, and any wellness operator implying equivalence with a hospital unit is misleading you.
The honest summary
Three tiers, and it is worth being blunt about all three.
Under 1.4 ATA.
Below the professional definition of HBOT. 1.3 ATA on air is the standard research placebo. Evidence for benefit at this pressure is thin, and where trials exist they have largely been negative. It is safe, it is pleasant, it is an hour of rest. It may well do something. Nobody has demonstrated that it does.
1.4 to 2.0 ATA.
Meets the professional definition. 2.0 ATA is where nearly every positive wellness-adjacent study sits. The evidence at this pressure is genuinely promising and genuinely incomplete: small samples, few sham controls, limited independent replication. Worth doing. Not worth believing uncritically.
2.4 ATA and above, in hospital.
Real medicine, real evidence, real indications, real supervision. Not available as a wellness purchase, and it should not be.
If you are choosing where to spend an hour and some money, the question to ask any provider is not “what are your benefits.” It is:
What pressure do you run, and can I see the gauge?
If they cannot answer clearly, or the answer starts below 1.4, you now know what that means.
What we run
Our chamber in the Byron Bay Arts and Industrial Estate is a hard-shell, single-person unit operating from 1.0 to 2.0 ATA, delivering oxygen at 93 percent purity through a dedicated breathing system, with live oxygen and pulse monitoring throughout the session.
We used to run a mild chamber at 1.35 ATA. We wrote most of this article working out whether that was defensible, and concluded it was not. So we replaced it.
Sessions are one hour and $49.99 until the end of 2026. Book one and see for yourself.
Curious what else is on offer? Browse our full range of recovery treatments.
We are a wellness club, not a medical clinic. Our sessions are not intended to diagnose, treat, cure or prevent any disease or medical condition. If you have a health condition, talk to your doctor first.
References
Undersea and Hyperbaric Medical Society. HBO2 Indications, 13th Edition. https://www.uhms.org/resources/hbo-indications.html
Undersea and Hyperbaric Medical Society. Definition of hyperbaric oxygen therapy. https://www.uhms.org/resources/hbo-indications.html
Mitchell SJ, Bennett MH. Unestablished indications for hyperbaric oxygen therapy. Diving and Hyperbaric Medicine 2014;44(4):228-34. PMID 25596836. https://pubmed.ncbi.nlm.nih.gov/25596836/
Wolf G, Cifu D, Baugh L, Carne W, Profenna L. The effect of hyperbaric oxygen on symptoms after mild traumatic brain injury. Journal of Neurotrauma 2012;29(17):2606-12. PMID 23031217. https://pubmed.ncbi.nlm.nih.gov/23031217/
Cifu DX, Hart BB, West SL, Walker W, Carne W. The effect of hyperbaric oxygen on persistent postconcussion symptoms. Journal of Head Trauma Rehabilitation 2014;29(1):11-20. PMID 24052094. https://pubmed.ncbi.nlm.nih.gov/24052094/
Hadanny A, et al. Cognitive enhancement of healthy older adults using hyperbaric oxygen: a randomized controlled trial. Aging (Albany NY) 2020;12(13):13740-13761. PMID 32589613. https://pubmed.ncbi.nlm.nih.gov/32589613/
Hachmo Y, et al. Hyperbaric oxygen therapy increases telomere length and decreases immunosenescence in isolated blood cells: a prospective trial. Aging (Albany NY) 2020;12(22):22445-22456. PMID 33206062. https://pubmed.ncbi.nlm.nih.gov/33206062/
Efrati S, et al. Hyperbaric oxygen therapy can diminish fibromyalgia syndrome: prospective clinical trial. PLoS One 2015;10(5):e0127012. PMID 26010952. https://pubmed.ncbi.nlm.nih.gov/26010952/
Zilberman-Itskovich S, et al. Hyperbaric oxygen therapy improves neurocognitive functions and symptoms of post-COVID condition: randomized controlled trial. Scientific Reports 2022;12(1):11252. PMID 35821512. https://pubmed.ncbi.nlm.nih.gov/35821512/
Heyboer M, et al. CD34+/CD45-dim stem cell mobilization by hyperbaric oxygen: changes with oxygen dosage. Stem Cell Research 2014;12(3):638-45. PMID 24642336. https://pubmed.ncbi.nlm.nih.gov/24642336/
Hu Z, Guo W, Wu H. Effects of acute mild hyperbaric oxygen exposure on cardiac autonomic function and aerobic performance. Journal of Physiological Anthropology 2025;44(1):22. PMID 40676637. https://pubmed.ncbi.nlm.nih.gov/40676637/
Cifu DX, et al. (see reference 5)
Kranke P, et al. Hyperbaric oxygen therapy for chronic wounds. Cochrane Database of Systematic Reviews 2015;(6):CD004123. PMID 26106870. https://pubmed.ncbi.nlm.nih.gov/26106870/
Lin ZC, et al. Hyperbaric oxygen therapy for late radiation tissue injury. Cochrane Database of Systematic Reviews 2023;8(8):CD005005. PMID 37585677. https://pubmed.ncbi.nlm.nih.gov/37585677/
Bennett MH, et al. Hyperbaric oxygen for idiopathic sudden sensorineural hearing loss and tinnitus. Cochrane Database of Systematic Reviews 2012;10:CD004739. PMID 23076907. https://pubmed.ncbi.nlm.nih.gov/23076907/
Mathieu D, Marroni A, Kot J. Tenth European Consensus Conference on Hyperbaric Medicine. Diving and Hyperbaric Medicine 2017;47(1):24-32. PMID 28357821. https://pubmed.ncbi.nlm.nih.gov/28357821/
