Why do Scuba Divers not use Pure Oxygen?

Why do Scuba divers not use pure Oxygen?

At depths below around 6m, pure Oxygen becomes toxic. The further divers go below this depth the more likely they will sucumb to Oxygen Toxicity or Central Nervous System Oxygen Toxicity (CNS) which can lead to convulsions and death.

Oxygen – it’s the one element all land animals that have evolved on Earth require almost all the time to function. It’s breathed in every second and transported around the body to fuel our muscles, our organs and most importantly our brain.

The more we need our body and our muscles to work, the more our body needs Oxygen, so we increase our intake by breathing harder and our heart starts pumping it around our bodies to fuel our cells.

Most of us know and experience this every day and most know the air we breathe, indeed our atmosphere maintains approximately 21% of the good stuff called Oxygen (O2).

Most of us should also know that if we start to lack available O2 to breathe, say we are at a higher altitude or stuck in a room with poor ventilation – we struggle to breathe, our minds can get fuzzy and at some point, we will pass out.

So what about the opposite?

  • What if we were able to breathe pure O2, would we be faster and stronger?
  • And what about diving, wouldn’t breathing pure O2 allow us to stay longer underwater?
  • Would it make us less tired as we are not breathing in Nitrogen (N2) as well?

The answer is a general no…with some exceptions.

O2 is a very reactive gas, meaning it likes to readily bind to other elements or compounds and change their nature. Rust on metal is a good example of O2 being corrosive and while our bodies have evolved to breathe this corrosive gas, sometimes too much of a good thing can prove to be another case.

Pure O2 is administered for medical reasons every day and at the surface, at one atmosphere of pressure, there is little problem with this. As N2 makes up a large amount (78%) of the air we breathe, we take in a lot in each breath.

Our bodies don’t use this N2 but they do store it in our muscles, bones, blood and cavities – absorbed into tissues.

When breathing pure O2 at sea level all that really happens is some of the N2 will seep out of our bodies through our blood and lungs, but this is minimal and will return to normal once we start breathing air again with no noticeable effect.

Under pressure, however, increased O2 can be beneficial by reducing the amount of N2 our bodies take in and usually it’s the N2 that can cause issues with our bodies and can lead to decompression sickness (DCS), which is why Enriched Air or Nitrox diving is popular.

But this goes only so far.

The higher percentage of O2 we breathe, the more likely we can succumb to Oxygen Toxicity or Central Nervous System Oxygen Toxicity (CNS) which can lead to convulsions and death.

Technically we can breathe pure O2 down to a depth of 6m, after that if we want to go deeper, we have to reduce the O2 percentage or we could suffer from CNS.

A 6m dive is not very deep and the margin for error is too slim to make it practical.

Technical divers who use re-breathers and go down far deeper than recreational divers – do use pure O2, however, it is mixed with Carbon Dioxide (CO2) and Helium (He) when breathed, reducing the percentage of O2 and making it nontoxic again.

Pure O2 is also expensive, can be a major fire hazard and it seems can kill us if we go just a few meters below the surface breathing it.

So while it has many uses for medical treatment or technical diving, breathing pure O2 for SCUBA is not something we can possibly do.

Diver looking at fish

Why does Oxygen Become Toxic at Depth?

At depth, Oxygen is converted to the more reactive, superoxide (an Oxygen molecule which has lost one electron) at a faster rate than at the surface. This highly reactive molecule binds particularly easily to vascular tissues, damaging them, causing organ systems to shut down.

As we have seen, pure O2 at depth is toxic, but what actually is the process that makes it happen?

O2 is used by our bodies to fuel our cells via the blood, in this process body’s metabolism reacts with the O2 molecules and some of them react by losing an electron, making them more reactive.

These are called superoxide anion or O2. When pressure is increased, these O2 molecules increase and begin to bind to our cells in certain places, particularly vascular tissues and quickly damage these cells.

The more O2, the more damaged cells, the quicker our bodies will start shutting down.

The O2 can also bind with Hydrogen (H) forming a product called hydroxyl radical (OH) which also damaged cells and tissues. The process is not completely understood, but it appears there are a few things happening to our bodies that can cause major harm.

What are the Symptoms of Oxygen Toxicity?

When the body starts to succumb to the O2and OH radicals it’s the central nervous system, lungs and eyes that are affected first and begin to show symptoms. Vomiting, dazzling in the eyes, lip twitching and nausea are all the first affects you will feel. If the toxicity is allowed to continue, vertigo, arm twitching, spasmodic respiration, confusion and death will follow. 

What can Pure Oxygen be used for?

As mentioned above, pure O2 at sea level, or one atmosphere is used in medical treatment in various ways.

In diving pure Oxygen is used as a first-aid treatment for anyone who has suspected decompression sickness. Even if a diver has some tingles in the hands or feet, feels they ascended too fast or they literally ascended faster than air bubbles; O2 is used to ‘flush’ any excess N2 out of the body that may have come out of your tissues. 

O2 can be used in technical diving where it is mixed in with other gases such as Carbon Dioxide and Helium in certain concentrations at certain depths to allow a technical diver to go far deeper than a recreational diver could.

Don’t Forget Your Dive Insurance!

Before you go out on any dive trip or holiday, it is essential to make sure you have insurance that covers you if something goes wrong. Check out our dive insurance article for more information.

Or go straight to these dive insurance company websites:


Diver Alert Network (DAN)

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While we may think that breathing pure O2 can help us in our diving by reducing the N2 we take in, this is not the case. O2 itself becomes toxic at certain levels and can start attacking our body’s cells instead of fueling them.

So by all means dive using enriched air or move into the technical diving realm and gain a better understanding of how increasing and decreasing different gases we breathe can affect you underwater times and depth, but don’t ever attempt to dive with pure O2 as the consequences could be quite tragic.

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Paul Fulbrook

Paul Fulbrook is a writer, scuba diver, ex-science teacher and marine biologist. He has a passion for coral reef biology, diving on coral reefs and writing about diving. He also loves cats and his children (sometimes).

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