Understanding ATA: The Pressure Unit at the Heart of HBOT

What ATA Measures

ATA stands for atmospheres absolute. One ATA is normal atmospheric pressure at sea level — about 14.7 psi or 760 mmHg. Hyperbaric medicine uses ATA to express chamber pressure relative to sea level: a chamber at 2.0 ATA is at twice sea-level pressure; one at 2.4 ATA is at 2.4× sea-level pressure.

Common HBOT Pressures

• 1.3 ATA — Soft-shell "mild HBOT" chambers. Only FDA-cleared for altitude sickness.
• 1.5 ATA — Entry-level clinical pressure; used in paediatric and veterinary protocols and some neurological research.
• 2.0 ATA — The pressure of most Efrati-lab neurological HBOT research (TBI, stroke, long COVID, fibromyalgia).
• 2.4 ATA — The UHMS clinical standard for wound care and most FDA-approved indications. The pressure CMS reimbursement is pegged to.
• 2.8–3.0 ATA — Emergency indications: carbon monoxide poisoning (Weaver protocol), decompression sickness (US Navy Table 6).

Why Pressure Matters

The clinical effect of HBOT scales with arterial oxygen tension (PaO₂). Breathing room air at 1 ATA produces a PaO₂ around 100 mmHg. Breathing 100% oxygen at 2.0 ATA pushes PaO₂ above 1,000 mmHg — a ten-fold increase. The extra dissolved oxygen in plasma drives angiogenesis, HIF-1α stabilisation, and tissue repair.

At 1.3 ATA with ~95% oxygen, the PaO₂ increase is much smaller. This is a key reason the clinical evidence for 1.3 ATA is weaker than for 2.0+ ATA protocols.

ATA vs ATG vs psi

Three closely related units often create confusion:

• ATA (atmospheres absolute) — pressure relative to a true vacuum. The standard in medical hyperbarics.
• ATG (atmospheres gauge) — pressure relative to atmospheric. 1 ATG = 2.0 ATA.
• psi (pounds per square inch) — US engineering unit. 1 ATA ≈ 14.7 psi.

Most clinical HBOT literature uses ATA. Diving tables and some engineering documents use ATG or psi. Always confirm the unit when reading protocol specifications.

Converting Between Pressures

Quick reference:

• 1.3 ATA ≈ 4.4 psi above atmospheric
• 2.0 ATA ≈ 14.7 psi above atmospheric
• 2.4 ATA ≈ 20.6 psi above atmospheric
• 2.8 ATA ≈ 26.5 psi above atmospheric
• 3.0 ATA ≈ 29.4 psi above atmospheric

The Practical Takeaway

When reading HBOT literature or evaluating a clinic, pressure is the single most important specification. A trial at 1.3 ATA is a very different intervention from one at 2.0 ATA. A clinic claiming "medical HBOT" at 1.3 ATA is operating outside the UHMS medical HBOT framework. Pay attention to the number.