Walk into any conversation about hyperbaric oxygen therapy and you’ll quickly hit a number: 1.3 ATA, 1.5 ATA, 2.0 ATA, sometimes 2.4 ATA. These numbers — measured in atmospheres absolute — describe how much pressure the chamber applies above sea level. They are the single most important variable in any HBOT protocol. Different pressures produce meaningfully different biological effects, are appropriate for different conditions, and carry different risk profiles. This is the complete guide to what each pressure does, who it’s for, and why 1.5 ATA has emerged as the consensus pressure for both home use and a significant portion of clinical use.
What "ATA" Actually Means
ATA stands for atmospheres absolute — a measure of total pressure including atmospheric pressure at sea level. At sea level, you’re already at 1.0 ATA. A chamber pressurized to 1.5 ATA delivers 1.5 times sea-level pressure, or roughly the pressure you’d experience 16 feet underwater. At 2.0 ATA, you’re at the equivalent of 33 feet underwater.
This is significant because pressure determines how much oxygen dissolves into your bloodstream — not just bound to hemoglobin, but free in plasma. The relationship is governed by Henry’s Law: the higher the pressure, the more gas dissolves into liquid.
Under normal conditions at sea level, breathing room air (21% oxygen), you carry about 0.3 mL of dissolved oxygen per 100 mL of plasma. Breathing 100% oxygen at 1.5 ATA, that number jumps to roughly 3 mL per 100 mL — a 10-fold increase. At 2.0 ATA, it’s about 4.5 mL. At 3.0 ATA (the upper limit of clinical HBOT), it can be 6+ mL — enough to keep tissue alive even without functional hemoglobin.
The biological consequence: pressurized oxygen reaches tissues that compromised circulation cannot. This is the entire mechanism of HBOT.
1.3 ATA: The Entry-Level Pressure
1.3 ATA is the pressure used in most “mild HBOT” or “mHBOT” chambers, including many home soft-shell chambers. At this pressure, you’re approximately 30% above sea-level pressure.
### What 1.3 ATA Does
– Increases dissolved oxygen by roughly 50% over sea level (when breathing concentrated oxygen)
– Improves cerebral blood flow modestly
– Reduces inflammation
– Drives some stem cell mobilization, though less than higher pressures
– Still produces clinical signal in conditions like autism (Rossignol 2009)
### Who 1.3 ATA Is For
– Beginners to HBOT who want to start gently
– Children and elderly patients for whom higher pressures may be poorly tolerated
– General wellness and recovery users without specific clinical targets
– Patients with claustrophobia who need to acclimate before higher pressures
– Those with ear and sinus issues who can’t equalize at higher pressures
### Limitations
The clinical evidence at 1.3 ATA is real but modest. For serious conditions — TBI, PTSD, longevity protocols — most of the major peer-reviewed trials use 1.5 ATA or higher. 1.3 ATA is a starting point, not a destination, for most clinical use cases.
1.5 ATA: The Consensus Standard
1.5 ATA is the pressure that has emerged as the consensus default for both home users and a significant portion of clinical practice. It is the pressure used in:
– Harch et al. (2017) — The most-cited PTSD/TBI HBOT trial
– Rockswold et al. (2013) — Severe acute TBI in the ICU
– Most athletic recovery protocols (LeBron James, Phelps, UFC fighters)
– Most home protocol research for chronic conditions
### What 1.5 ATA Does
At 1.5 ATA breathing concentrated oxygen, dissolved oxygen in plasma rises 7–10 fold. This is sufficient to:
– Drive substantial stem cell mobilization (CD34+ cells double after a single session)
– Restore cerebral blood flow in injured brain regions
– Trigger angiogenesis (new blood vessel formation)
– Reduce neuroinflammation across the CNS
– Support telomere maintenance and senescent cell clearance over longer courses
### Why 1.5 ATA Won
The case for 1.5 ATA is essentially three-part:
1. Sufficient pressure to drive every major HBOT mechanism. The biology that responds to 2.0 ATA also responds to 1.5 ATA — the magnitude is slightly smaller, but the response is consistent.
2. Far better tolerability than higher pressures. No clinically meaningful oxygen toxicity at 60-minute sessions. Far easier to equalize ears. Lower drop-out rates in clinical trials.
3. Compatible with FDA-cleared home chambers. Soft-shell chambers safely operate at 1.3–1.5 ATA. The 2.0+ ATA range requires hard-shell hospital chambers, which cost $50K–$200K and are impractical for home use.
The result: 1.5 ATA is the only pressure where serious clinical efficacy and home accessibility intersect.
2.0 ATA: The Clinical Trial Pressure
Most of the Israeli longevity and brain health research from the Sagol Center uses 2.0 ATA. So do many wound care protocols and historic decompression sickness protocols.
### What 2.0 ATA Does
At 2.0 ATA, dissolved plasma oxygen reaches roughly 4.5 mL per 100 mL plasma — enough to support tissue viability without hemoglobin binding. Effects include:
– Strongest stem cell mobilization (8x baseline after 20 sessions)
– Maximal angiogenesis stimulation
– Most documented telomere extension (21.7% over 60 sessions)
– Highest documented senescent cell clearance
### When 2.0 ATA Matters
– Severe wounds that have failed standard care (diabetic ulcers, radiation injury)
– Acute carbon monoxide poisoning
– Decompression sickness (the original use case)
– Severe necrotizing infections
– Cognitive aging research protocols where the goal is maximizing telomere effects
### Limitations of 2.0 ATA
– Requires a hard-shell, FDA-approved hospital-grade chamber, which is rarely available outside specialized centers
– Higher rates of ear barotrauma in patients who can’t equalize well
– Oxygen toxicity becomes a real concern, especially with sessions longer than 90 minutes — this is why most 2.0 ATA protocols include 5-minute “air breaks” every 20 minutes
– Per-session clinic cost typically $300–$600 in the U.S.
For most home users and most chronic conditions, the marginal benefit of 2.0 ATA over 1.5 ATA does not justify the access barriers.
2.4 ATA and Above: Specialty Indications
2.4 ATA — sometimes 2.5 or even 3.0 ATA — is reserved for specific clinical indications where high-dose oxygen exposure is needed quickly:
– Decompression sickness (Treatment Tables 5 and 6)
– Arterial gas embolism
– Severe carbon monoxide poisoning
– Some refractory wound care cases
– Necrotizing fasciitis
These pressures are not used for chronic conditions or wellness. The risk profile (oxygen toxicity, barotrauma, seizure risk) is real, and the additional benefit beyond 2.0 ATA is marginal for most indications.
Why Oxygen Concentration Matters as Much as Pressure
Pressure determines how much oxygen *can* dissolve. Oxygen concentration determines how much actually does. Both variables work together.
A chamber pressurized to 1.5 ATA but breathing room air (21% oxygen) produces a much smaller effect than a chamber at 1.5 ATA breathing 95% oxygen via mask. Many older mHBOT chambers fall into this trap — they advertise pressure but deliver only ambient air. Modern best practice is 1.5 ATA combined with 90–96% oxygen at the user’s mask, fed by a quality 10 LPM oxygen concentrator (or two concentrators in parallel for higher flow).
This combination produces a roughly 7–10x increase in dissolved plasma oxygen, which is the actual therapeutic dose. Without the concentrator, the same 1.5 ATA chamber delivers only a fraction of the biological effect.
Hard-shell hospital chambers can deliver 100% medical oxygen safely because their rigid construction allows for fire suppression engineering. Soft-shell chambers cap at roughly 96% oxygen for safety reasons, but the marginal difference between 96% and 100% is small at the dosing levels HBOT uses.
Choosing Your Pressure: A Practical Framework
For most users designing a personal protocol, the decision tree looks like this:
### Start at 1.3 ATA if you are:
– New to HBOT and unsure about tolerance
– Treating a child or elderly patient
– Using HBOT for general wellness without specific clinical targets
### Use 1.5 ATA if you are:
– Treating chronic TBI, post-concussion syndrome, or PTSD
– Running a longevity protocol at home
– An athlete using HBOT for recovery
– Treating chronic Lyme, fibromyalgia, or post-viral fatigue
– Looking for the strongest clinical evidence with home accessibility
### Use 2.0+ ATA if you are:
– Working with a clinical team in a hospital-grade hard-shell chamber
– Treating a wound care or decompression case
– Running a research-grade longevity protocol with proper monitoring
The most common mistake is staying at 1.3 ATA when 1.5 ATA would be more appropriate. The second most common is jumping to 2.0+ without proper supervision. For the vast majority of evidence-based use cases, 1.5 ATA at 95–100% oxygen for 60 minutes is the right answer.
Pressure is the foundational variable in HBOT, but it’s not the only one. Oxygen concentration, session length, total session count, and frequency all interact with pressure to determine your actual biological dose. The major clinical trials cluster around 1.5 ATA for chronic conditions and 2.0 ATA for cognitive aging — both with 60-minute sessions, 95–100% oxygen, and 40–60 total sessions. If you are designing a home protocol, those numbers are your starting point, not 1.3 ATA. To build your full protocol — adjusted for your specific condition, experience, and time commitment — open the Protocol Calculator. It pulls directly from the studies referenced in this article.