A significant debate exists within the hyperbaric medicine community regarding the optimal atmospheric pressure for treatment, with particular controversy surrounding whether 1.3 ATA (atmospheres absolute) represents an inert placebo or an active therapeutic intervention.​

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The Core Disagreement

The controversy centers on study design and what constitutes "sham" treatment. Historically, many researchers used 1.2-1.3 ATA with room air (21% oxygen) as a control/sham condition in clinical trials, operating under the assumption that pressures below 1.4 ATA with air would be therapeutically inert. However, this assumption is increasingly challenged by evidence suggesting that even mild hyperbaric pressures produce biological effects.

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The Problem with "Sham" Controls

When studies use 1.3 ATA as the baseline or sham condition and report inconclusive results, some researchers argue this may be because both groups are actually receiving active treatment. Breathing regular air at 1.3 ATA is equivalent to approximately 27-28% oxygen at normal atmospheric pressure, resulting in more than 50% elevation in tissue oxygenation. This creates a fundamental problem: the "placebo" group may be experiencing therapeutic benefits, masking the true effectiveness of higher pressure protocols.

Multiple editorials published in Undersea and Hyperbaric Medicine have "vehemently condemned the use of hyperbaric pressurization as a valid intervention for a control group as it is not an inert intervention". Research has shown that even slight increases in partial pressure of oxygen, or pressure alone, can cause physiological changes in the body.

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Evidence Supporting Lower Pressure Efficacy

Several clinical studies support the therapeutic potential of lower pressures. A 2024 systematic review analyzing PTSD treatment demonstrated that reliable improvements occurred at pressures as low as 1.3 ATA, with a minimum pressure threshold for symptom improvement appearing at 1.2-1.3 ATA. The Wolf et al. study showed that subjects receiving 1.3 ATA with 21% oxygen experienced a statistically significant 8.3-point reduction in PTSD symptoms, nearly identical to the 8.4-point reduction seen in the 2.4 ATA 100% oxygen group.​

Similarly, the Miller et al. study found a clinically meaningful 11.4-point decrease in symptoms with 1.2 ATA treatment, compared to only a 5.0-point decrease with 1.5 ATA 100% oxygen. A 2021 validation study confirmed that compression to 1.3 ATA, when combined with proper blinding strategies, successfully simulates therapeutic compression and can be perceived by patients as full treatment pressure.

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The Case for Mild Hyperbaric Therapy

Advocates for lower pressure protocols emphasize three key advantages:

Safety: Lower pressures significantly reduce the risk of complications, particularly middle ear barotrauma and oxygen toxicity. Studies using 1.2-1.5 ATA report barotrauma rates as low as 2-5.5%, compared to 33-43% in some higher-pressure protocols.

Accessibility: Mild hyperbaric chambers operating at 1.3-1.5 ATA can utilize soft-shell portable chambers, dramatically reducing costs and increasing availability for patients. These systems can be deployed in outpatient settings, homes, and community facilities where traditional hard-chamber systems are financially prohibitive.

Efficacy through frequency: Rather than requiring higher pressures, conditions may respond to more treatment sessions at lower, safer pressures. The 2024 systematic review demonstrated a "near-linear dose-response curve for increasing PTSD symptom improvement with increasing total oxygen dose," suggesting cumulative exposures matter as much as pressure intensity.

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The Traditional High-Pressure Perspective

Critics of mild hyperbaric therapy argue that established protocols for conditions on the Undersea and Hyperbaric Medical Society's approved list typically require 2.0-2.5 ATA. They contend that the historical definition of HBOT as requiring ≥1.4 ATA with 100% oxygen exists because lower pressures and concentrations haven't demonstrated efficacy for approved indications like carbon monoxide poisoning, diabetic foot ulcers, and radiation tissue injury.

However, the FDA's reclassification of HBOT as consisting of both increased barometric pressure and hyperoxia as separate therapeutic components has complicated this debate, acknowledging that both elements contribute independently to treatment effects.​

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Implications for Research and Practice

This controversy has profound implications for clinical trial design and interpretation. When studies report "no significant difference" between treatment groups using 1.3 ATA as control, the results may actually demonstrate that both pressures work, not that neither works. As researchers have noted, 1.3 ATA "simply cannot be both" an active treatment and an inert control.

The debate ultimately reflects deeper questions about standardization, evidence thresholds, and accessibility in hyperbaric medicine, with significant financial and practical implications for patients seeking treatment.