What you need to know about Oxygen Toxicity

One of the most dangerous problems in diving is oxygen toxicity. It’s basically having too much oxygen when you are under water. It’s also called hyperoxia.

The air on land consists of around 21% O2 (oxygen). Without oxygen we’re not able to survive. However, too much oxygen can also be a bad thing. Yet, it’s not really the amount of O2 in the air that is of importance but the partial pressure of O2 (pO2). If that gets too high (or certainly too low) it’ll end up becoming toxic which can be fatal.

Oxygen Toxicity During DIving

What is pO2?

The partial pressure of O2 is measured in ATA (atmospheres). On the surface you have 0.21 ATA (21% [0.21] times 1 ATA). Our bodies can sustain an pO2 of 0.16 ATA on the lower end. When exposed often our body can even adapt to lower pO2 levels.

On the upper level we can tolerate pO2 levels of up to around 0.45 ATA for longer periods. During shorter periods this level can be increased to more than 1.6 ATA. Yet, if you are exposed to high levels like that for a longer time (hours or days) then there will experience toxic effects that will first affect your lungs.

However, if you’re exposed to levels greater that 1.6 ATA the toxic effects start at the brain. Even after just minutes of exposure! The danger is that one day you might not have any issues but your body can react the next day by showing those toxic effects!

How to calculate?

When you dive you will automatically be exposed to higher pO2 levels. Every 33 feet of depth results in an increase of 1 ATA of pressure. So, at 99 feet the pressure is 4.0 ATA. Diving with Air accordingly results in 4.0 * 0.21 which is 0.84. Quite a ways away from the limit of 1.6 we mentioned above.

The whole picture changes when we look at Nitrox. If we take a 40% O2 level in Nitrox then you’re already at 0.4 ATA on the surface. At 99 feet with 4 ATA of pressure you get to 4.0 * 0.4 equaling 1.6 ATA! You’re passing the threshold where it can get dangerous for your health at that depth!

The table below shows at what depths you’re dealing with too high levels of pO2 during diving.

Depth (feet)

Pressure (ATA)

pO2 (ATA) for Air (21% O2)

pO2 (ATA) for Nitrox (40% O2)

























Diving with Air

Most recreational divers that only dive with air pretty much never have to worry about oxygen toxicity. There’s basically no way that you’d reach depths below 218 feet to face any problems. Why 218 feet? At 218 feet the pressure reaches 7.61 ATA. If you multiply with 0.21 then you end up at the ‘magic’ 1.6 ATA of pO2 where things become a problem.

Realistically, you won’t dive deeper than 130 feet and as such stay way above the depth where you would be exposed to oxygen toxicity. At 130 feet you end up at just a tad over 1.0 ATA. At 132 feet the pressure reaches 5 ATA which results in 1.05 pO2 (5.0 * 0.21). At that level there should be nothing to worry about.

The air supply is also limited so there’s practically no chance to stay at even a 1.05 ATA pO2 that would be long enough to cause lung toxicity. You simply can’t bring enough air tanks with you to reach that limit!

Diving with Nitrox

Using Nitrox the calculations change dramatically. An increasing number of recreational divers use Nitrox with up to 40% O2. In some cases they might even use pure oxygen for decompression.

With these exposures to higher levels of O2 it can very quickly happen that you face oxygen toxicity. The table above shows that you reach a pO2 level of 1.6 ATA at only 99 feet! Compared to the 218 feet for Air, this is a depth that many recreational divers find themselves in regularly and long.

This results in a real danger to your health! If you dive with Nitrox then make sure to do your calculations upfront and to use a dive computer that alarms you when the pO2 limits are reached!

What damage can happen?

For the most part oxygen toxicity will affect the human brain and the lungs. Long-term exposure will damage your lungs. The cells will die off and your body won’t be able to absorb oxygen anymore which will lead to suffocation.

At pO2 levels between 0.45 ATA and 1.6 ATA the most impacted organ is your lung. Above a level of 1.6 ATA your brain will suffer.

What happens in your body?

Your cells in your body are built to be able to live within a certain range of pO2. Exposing them to higher levels for a long time simply kills them. If enough of your cells die then eventually you will die.

In most cases we simply are not exposed to high enough levels of pO2 or we’re not exposed long enough. The damage in those cases is reversible and as such usually doesn’t have a lasting impact.

As mentioned, the two organs most affected are the lungs and the brain. However, long exposure to medium levels of pO2 can impact another part of your body – the eyes. The effect is that your eye can start to become near-sighted. This is also called hyperbaric induced myopia.

Lung Oxygen Toxicity

Going above a pO2 level of 1.6 ATA will impact the brain before the lungs. Long exposure to elevated pO2 levels affect the lung and you will feel mild irritation in the throat. The deeper you breathe the worse that irritation will get!

The next stage is that you’ll experience a mild cough that increases continuously until it’s quite painful. At that time you usually cannot stop to cough anymore! If at that time the exposure continues you will feel tightness of the chest and difficulty in breathing. Shortness of breath also sets in. Continuing exposure will eventually cause death. The lungs cannot exchange CO2 for O2 anymore and your body will not get oxygen and you will basically suffocate.

Prevention of pulmonary (lung) oxygen toxicity is to simply limit the exposure to pO2 to the shortest time possible. Diving with air only and not diving below 130 feet should ensure that you’re not facing any issues.

When diving with Nitrox it can help to add breaks where you switch to air instead of nitrox. That will allow your body to repair at least some of the cell damage in your lungs during the period in which you switch to breathing air. A possible scenario would be that for every 25 minutes of Nitrox you breath at least 5 minutes of air. There’s no guarantee though that this will work to avoid oxygen toxicity of your lungs completely. It can provide you with a longer time period though before its onset.

Oxygen Toxicity of the Brain (CNS)

Damage to the brain can happen much quicker than for the lungs. However, you will need to be exposed to higher pO2 levels. When diving with air that’s practically not possible to happen.

With Nitrox the possibility exists. pO2 levels of 1.6 are reached at only 99 feet with a 40% oxygen level. Pure oxygen (100%) reaches this level at a depth of 20 feet! Be careful when you decompress on 100% oxygen!

Recreational divers using 40% Nitrox will usually not run a risk. The story is different for technical and commercial divers that go deeper and stay down longer on Nitrox!

The effects of CNS oxygen toxicity often come without warning. Some people experience a seizure with no other precursors like headaches, etc. While there is a good thing about seizures that come from brain oxygen toxicity there’s also a very bad outcome that is very real.

First, the good news (if we really want to call it that). Because your body and brain are having so much oxygen on board there’s most likely no permanent damage due to CNS oxygen toxicity when you experience such a seizure. The bad news is that because you have such a seizure there’s a very, very high chance that you will drown. Or, if you ascend there’s a chance that you suffer pulmonary barotrauma which also can be fatal!

The symptoms of CNS oxygen toxicity range from convulsions (seizures) to tunnel vision, ringing in your ears, nausea and dizziness. The seizures are obviously the worst effect and can end in death.

You cannot prevent CNS oxygen toxicity through the use of any drugs. You can only prevent it by limiting your exposure to pO2.


To avoid any possible implications you want to stay below a pO2 level of 1.6 ATA. Diving with air will usually not let you reach that level but using Nitrox can put you at risk.

Develop a dive plan and stick to it. Make sure you stay below 1.6 ATA and even reaching that level should be for the shortest possible time. It’s better for your health while you’ll still be able to enjoy the wonders under water!

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