Is breathing in pure oxygen harmful? The short answer to this question is, ‘YES.’ But why? We will try to answer the question in the simplest possible way.

Breathing in 100% oxygen or pure oxygen is never a good idea and you should know this. It can not only be damaging to our lungs but also our brain and heart.

Let us find out what happens to us when we breathe in pure oxygen.

The Air That We Breathe

Do you know how much oxygen is present in the air that we breathe in? It contains 21% oxygen. The rest is a mixture of other gases like nitrogen, carbon dioxide, etc.

It is that way for a reason. Nature could have, if she wanted to, give us 100% oxygen to breathe in. She doesn’t do that and there are very good reasons for that. After all, there has been no greater scientist ever than Mother Nature!

The Lungs

Our lungs are a long series of tubes branching out from the nose and the mouth. The branching happens from the trachea to bronchi to bronchioles. The bronchioles eventually end up in alveoli. Alveoli are tiny air sacs with very thin walls.

To imagine how bronchioles end up in alveoli, think of straw with several soap bubbles at the end of it. You are getting the image, right?

A single air sac is called alveolus and all the air sacs are together called alveoli.

Each alveolus is surrounded by blood vessels, which too, are thin-walled. These blood vessels are called pulmonary capillaries.

Between the alveolus and the blood vessels or the pulmonary capillaries is yet another wall. This wall is very thin, measuring only 0.5 microns in thickness. Just to give you an idea, one micron is one-millionth of a meter. So, 0.5 microns is half of that!

This super thin wall between the alveolus and the blood vessels is the one through which different gases pass. What gases? Gases like nitrogen, carbon dioxide, and oxygen.

What Happens When You Breathe in Air?

When you breathe in air, that is, when you inhale, the alveoli fill up with the air that contains 21% oxygen. On the other hand, blood enters the pulmonary capillaries.

The blood entering the pulmonary capillaries is deficient in oxygen (rich in carbon dioxide) but the air entering the alveoli is rich in oxygen.

So, the oxygen from the alveoli diffuses into the blood because the oxygen concentration in the blood is low.

On the other hand, the carbon dioxide from blood diffuses into the alveoli because the carbon dioxide concentration in the alveoli is low.

This exchange of gases by diffusion takes place through the wall (of 0.5 microns thickness) between the alveoli and the pulmonary capillaries.

Once the exchange of gases is completed, the air in the alveoli becomes rich in carbon dioxide and the blood in the pulmonary capillaries becomes rich in oxygen.

The nitrogen concentration remains more or less the same in both in alveolar air and the blood.

So, when you breathe out, that is, when you exhale, the air that you release is rich in carbon dioxide. The oxygen-rich blood in pulmonary capillaries then travels back to the heart from where the blood is pumped to the rest of the body, supplying oxygen.

Breathing in Pure Oxygen – What Happens?

Let us answer this question with the help of an experiment that was conducted back in 2007. In that experiment, researchers took 14 healthy children within the age group of 8 and 15.

The children were made to inhale 100% pure oxygen for two minutes straight with the help of mouthpieces.

After that, the researchers waited for 8 minutes before giving them the mouthpieces but only this time, pure oxygen was mixed with 5% carbon dioxide.

In both instances, the children were scanned using functional magnetic resonance imaging. Essentially, their brains were scanned for the changes.

Some surprising results came in.

Results after breathing in pure oxygen

  • Breathing in children quickened after they were given pure oxygen.
  • There was rapid exhalation of carbon dioxide from their bodies.
  • As the carbon dioxide levels dropped, their blood vessels narrowed.
  • Narrowed blood vessels, in turn, prevented the oxygen from properly reaching the brain and the heart.
  • It MRI scan it was seen that the hippocampus, insula, and cingulate cortex lit up. Hippocampus is responsible for blood pressure control. The insula is responsible for monitoring physical and emotional stress. The cingulate cortex is responsible for the regulation of blood pressure and pain perception.
  • As the different activities in the brain took place, the hypothalamus responsible for regulating hormonal outflows and heart rate woke up.
  • The hypothalamus started overreacting by dumping enormous amounts of neurotransmitters and hormones into the bloodstream. These chemicals in the bloodstream interfere and negatively impacts the heart’s ability to pump blood and deliver oxygen.

Results after breathing in oxygen mixed with 5% carbon dioxide

  • The hypothalamus that was in overdrive because of inhalation of pure oxygen now calmed down. The hyperactivity simply vanished and enormous amounts of chemicals were no longer dumped into the bloodstream.
  • The blood vessels also relaxed, allowing oxygen to reach the brain and the heart properly.

The study thereby concluded that breathing in pure oxygen can over time damage the brain and the heart over the long run.

What Else Happens When You Breathe in Pure Oxygen?

Heart and brain are not the only ones to suffer. There is a lot more that happens!

Please understand that all this that you read till now was all done under normal pressure. If you keep breathing in pure oxygen at normal pressure, a lot more happens.

  • Fluids start accumulating in the lungs.
  • There is a slowdown of gas flow across the alveoli. This means that you have to breathe more to get enough oxygen.
  • During deep breathing, you will experience chest pains.
  • In the lungs, there is a 17% in the total exchangeable gas volume.

The alveoli collapse and lead to a situation called atelectasis in which, local areas of collapsed alveoli are plugged by mucus.

Whatever oxygen is left in the alveoli plugged by mucus gets absorbed by the bloodstream, leaving behind no gas that can keep the plugged alveoli inflated and they eventually collapse completely.

Instead of pure oxygen if you breathe in normal air and the alveoli get plugged by mucus, the nitrogen present in the alveoli keeps them inflated and prevents them from completely collapsing.

But all the problems stated above occur when you keep breathing in pure oxygen for more than 24 hours at normal pressure. Apart from that, long term use of pure oxygen will rapidly generate free radicals of oxygen.

Free radicals are highly unstable (unlike the molecular oxygen which is very stable) and are also highly reactive.

These free radicals will try to stabilize themselves by reacting with intracellular components like lipids, proteins, DNA, etc., eventually damaging them and making them non-functional, and eventually, the cells (with which the free radicals interact) die.

The free oxygen also impacts the central nervous system and even attacks the retina!

But, why do you think breathing in people increases when they breathe in 100% oxygen?

The answer to this question can be found in the process of free radical formation. Initially, when free radicals are formed, the antioxidants present in our body are adequate to neutralize those free radicals.

Unfortunately, however, at one point the balance tips in favor of the free radicals because their production keeps on increasing. At one point, the supply of free radicals simply overwhelms the antioxidation capacity of the body, leading to oxidation stress.

This oxidation stress leads to inflammation in the tissues of the lungs and hinders the diffusion of oxygen from the alveoli into the blood through the alveolar wall. This impedes the uptake of oxygen, leading to increased breathing!


Breathing in pure oxygen under normal pressure for a short period will not impact at all. When you keep breathing in pure oxygen for an extended time (more than 24 hours), all the harmful effects start setting in.

Likewise, if you inhale pure oxygen at lowered pressure, no damage will be caused for a longer period.

A good example of this is the astronauts of Apollo and Gemini programs who inhaled 100% oxygen for up to 2 weeks but at lowered pressure.

On the contrary, if you inhale 100% oxygen at 0.5 bars (2.5 times the normal atmospheric pressure) or more for just 16 hours, it will lead to irreversible lung damage, and eventually death!

This scenario of inhaling pure oxygen at increased pressure leads to acute oxygen poisoning with some severe symptoms like convulsions and seizures, blurred vision, muscle twitches, dizziness, nausea, etc.

So, contrary to popular belief, breathing in pure oxygen for some time under normal pressure will not lead to oxygen toxicity. The dizziness that occurs is only a result of carbon dioxide levels dropping too low!



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