Oxygen Tolerance – Myth Vs Fact – An Introduction To a Three Part Series

Photo by Pixabay on Pexels.com

Medical professionals will notice I head this article “Oxygen Tolerance” as opposed to “Oxygen Toxicity” as it has more commonly been known. Most medical doctors consider excess oxygen toxic to humans. This is partly why administration of hyperbaric oxygenation is largely misunderstood and accordingly, avoided in mainstream medicine. Many doctors unfamiliar with HBOT will almost universally shake their heads and mumble something about toxicity when the subject is raised. It boils down to misconceptions of the seriousness of what most refer to as oxygen toxicity. And this stems from a lack of updated information being disseminated in the instruction of new medical professionals.

The oxygen toxicity mechanism is not extensively taught outside of post graduate modules on hyperbaric medicine when it comes to medical instruction. It is however taught to divers and supervisors, during chamber operator training, diver training, supervisor instruction and is also found in deep diving instruction literature. Most of which is taken from and inspired by the divers “bible” so to speak, The United States Navy Diving Manual which is respected as a global standard.

US Navy Diving Manual Rev.7A

It is also very much misunderstood from one medical discipline to another with the standard line of, “the mechanism is not fully understood”, being the party line. It is a subject seemingly better covered in diving than it is in medicine.

Not to disagree entirely though, oxygen in excessive quantities and at very high pressures can be harmful indeed, and it can have undesirable side effects. There is merit in caution and proper management of the actual risk. Unfortunately there seems to be a level of higher perceived risk which takes the lead. The term “fake news” comes to mind. It is true that the exact mechanisms of this toxicity aren’t fully understood, but, perhaps better understood by divers more comprehensively than by medical staff. No disrespect is intended, but it must be acknowledged that it is an integral part of commercial, military and technical diver and supervisor training. And not simply appointed supervisors, Properly certified hyperbaric and diving operations supervisors in the military sense. It should be noted that whether being considered for diving alone or HBOT, the basic principal of oxygen toxicity remains the same for both disciplines its just the limits and contributing environmental factors that differ.

A better understanding of this is desperately needed across both medical and diving professions today. Although, having said that since the original writing of this material, studies have shown that various intolerance, or toxicity response, specifically cerebral intolerance, to be related to the lowering of glucose on the other side of the blood brain barrier (BBB), essentially meaning that any neurological signs observed are caused by what could be described a “diabetic” event, brought about by the lowering of neurological glucose levels. In any event these signs leave no residual sequelae and cease when oxygen breathing ceases. (David F.WilsonFranz &M.Matschinsky Nov 2019 -‘Hyperbaric oxygen toxicity in brain: A case of hyperoxia induced hypoglycemic brain syndrome.’)

This will be discussed further in the second part of this three part series covering Central Nervous System toxicity or CNS/Cerebral Toxicity.

Personally, we subscribe to and employ the Repex method (which too is based on the “Haldanean Effect” mentioned below), for determining exposure limits. The Repex method describes something called “whole body toxicity”, rather than separating cerebral and pulmonary toxicity. We, among many others, call it “tolerance”, in keeping with the authors of the Repex method of determining biological tolerance levels. (Tolerating Exposure to High Oxygen Levels: Repex and Other Methods – 1989 http://www.tecvault.t101.ro/REPEX0.PDF )

Medical science, and indeed most doctors, refer to oxygen as a poison or a drug. This is wholly untrue. Oxygen can merely behave in the manner a poison behaves under certain specific conditions. Left alone it can hardly be considered toxic. It’s everywhere with no ill effect. It is a naturally existing compound with absolutely zero narcotic value which doesn’t exhibit the pharmacology that “drugs” are described as having. For this reason it is currently not regulated as a drug.

How is it, that the only gas that can safely be breathed as pure gas is regarded a poison? And how is that the one compound singularly essential to life can be called a poison? Simply, it cannot. It is not. It’s in the air we breathe daily.

Image by Kathleen Port from Pixabay

Similar can be said of vitamin A or just about anything we consume. Too much of almost anything could harm one. Historically oxygen toxicity was only ever a problem to people exposed to either high concentrations of oxygen, or to those exposed to lower concentration, but with these lower concentrations over a long period of exposure. These principals still apply today but the specific level of tolerance is where the disagreement is most evident with some imposing un-realistic limits on how much oxygen can be safely administered. To complicate it further, intolerance differs from person to person and even from day to day in the same person. There is no absolute value that can be universally applied. Despite that, universal limits are applied.

These universally applied limits as they are applied today, are incidentally still based on the original experiments conducted by the aforementioned J.S. Haldane on miners who were asked to run on a treadmill breathing pure oxygen. It turned out that after running on a treadmill for a given time breathing pure oxygen, the subject would suffer a convulsion (cerebral toxicity). Ordinarily a harmless event save for bumping a head accidentally.

Accordingly, Haldane lowered the oxygen concentration in further versions of the experiment, without considering the physical exercise involved, and after subjects breathing just 60% oxygen, no convulsion occurred. He concluded rather simplistically that a lower level of oxygen was the way to avoid toxic response. One which unfortunately persists today. Again he discounted the psychical exertion as a key contributor to the toxic response, which it is.

This is where today’s hospital recommendation of breathing only 60% oxygen comes from. This is also where the arguably irrational fear of oxygen toxicity comes from. What hasn’t been accounted for is the exertion. I find that most patients don’t run on treadmills and it is exertion that drastically affects tolerance.

In diving, it is agreed, especially for commercial and military divers who work physically hard at depth, toxicity tolerance levels should lower than for a resting person in a chamber or hospital bed. Even recreational diving employs very conservative exposure limits for in water applications. And rightly so. The water itself also affects susceptibility to toxicity response.


It seems rather unnecessary though to apply diving limits to a safe and dry resting environment like a chamber at the expense of a viable, effective and cost effective modality.


What seems to have happened is the same “treadmill miner” approach and mindset has been universally applied which would be erroneous.

The two types of oxygen toxicity that concern us in diving and HBOT are namely, “Cerebral Toxicity” or “Central Nervous System Toxicity” (CNS) and “Pulmonary Toxicity”.

These two sub category topics, as well as the single method approach of whole body toxicity, as promulgated by the main reference above (The Repex Method), will be discussed in the next two installments of the toxicity and tolerance discussion.

Remember this in the meantime though. It’s not necessarily what has been taught for over a hundred years and this has been proven time and again in HBOT therapy. Toxic response is not as common and as easy to induce as many would have you believe. The current official position is very much an overstated one in many ways.

Part two to follow will address cerebral or central nervous system toxicity and part three will address pulmonary toxicity in a responsible realistic way to highlight what risk is real and inherent, and highlight where this risk has been overstated by some.

©Hayden Dunstan

Image by joakant from Pixabay 

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