Should you breathe through your nose or mouth during exercise?

“You should breathe through your mouth when exercising as it allows you to take in more oxygen to fuel your muscles.”


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“You should breathe through your mouth when exercising as it allows you to take in more oxygen to fuel your muscles.”

This advice posits that bigger breaths result in higher blood oxygen saturation levels which, theoretically, improves muscular strength and endurance. It sounds obvious, doesn’t it? Your mouth is bigger and less likely to be obstructed compared to your nose, so it makes sense why mouth inhales should allow you to take in a larger volume of oxygen per breath. Similarly, mouth exhales allow you to expel every bit of that ‘toxic’ carbon dioxide that everyone warns you about. It’s a well-intentioned piece of advice … but woefully misguided and potentially harmful.

In this article, I outline the pitfalls of mouth breathing, detail the advantages of increasing carbon dioxide levels in your blood, and assert the benefits of nasal breathing in bolstering athletic performance.

 What is Oxygen Saturation and why does it matter?

Let’s start with some background information. The oxygen in the air that we breathe in is metabolised by our muscles to work efficiently. Oxygen saturation refers to the percentage of oxygen-carrying red blood cells that contain oxygen. An average person breathes about four to six litres of air per minute, resulting in oxygen saturation levels of approximately 95% to 99%.

Why not 100%? An oxygen saturation of 100% implies that the bond between red blood cells and oxygen molecules is too strong, inadvertently hampering the former’s ability to release oxygen to muscles, organs and tissues for metabolisation. In short, increasing oxygen saturation to 100% has no added benefits.

At any point in time, a healthy body carries a surplus of blood oxygen (>95%) even during periods of intense physical activity. You may in fact validate this through taking pulse oximetry readings during your next high intensity training session. Deliberately increasing oxygen levels is like pouring more water into a glass that’s already filled to the brim. So why do people still believe that it’s necessary to take large breaths to further increase blood oxygenation?

One reason may be the discomfort of breathlessness we experience when we partake in vigorous activities. Or when we’re having an asthma attack. The panting and wheezing suggest that there’s a shortage of oxygen coupled with an excessive build up of carbon dioxide in the body. Almost immediately, we take large breaths to correct this unbalanced state. As mouth breathing feels less obstructive, that’s the way most of us breathed in response.

And that’s where the problems begin.

The Bohr Effect and Hypocapnia

 In the words of Danish physiologist Christian Bohr, “The carbon dioxide pressure of the blood is to be regarded as an important factor in the inner respiratory metabolism. If one uses carbon dioxide in appropriate amounts, the oxygen that was taken up can be used more effectively throughout the body.”

Haemoglobin, a protein found in blood, carries oxygen from the lungs to tissues and cells. The crucial point, based on Bohr’s findings, is that haemoglobin releases oxygen when in the presence of carbon dioxide. When you take large breaths through the mouth, not only are you taking in high volumes of unprocessed and unfiltered air, too much carbon dioxide is also washed from the lungs, blood, tissues, and cells when you exhale. This decrease in blood carbon dioxide levels below the healthy range, hypocapnia, causes haemoglobin to hold tightly onto oxygen, thus reducing oxygen delivery to tissues and organs. As counterintuitive as it feels, bigger and more frequent breaths through the mouth when we ‘hit the wall’ do not provide the muscles and lungs with more oxygen, but actually reduces oxygenation even further. Hyperventilation begets more hyperventilation.

The Advantages of Nasal Breathing

Conversely, as you breathe through your nose, the gases are moved through scrolled, spongy turbinates which guide the inhaled air. The internal cavity regulates the direction and velocity of the air to maximise exposure to the vast network of arteries and veins. The mucus blanket warms, humidifies, and sterilises the air before it goes to the lungs. Due to these processes that occur within the nasal cavity, air inhaled through the nose encounters around 50% more resistance to the airstream - but results in 10% to 20% more oxygen uptake!

In addition, nitric oxide - an essential gas for vasoregulation, homeostasis, neurotransmission, immune defence, and respiration - is produced inside the nasal cavity and the lining of blood vessels laden throughout the body. Nitric oxide combats high blood pressure, lowers cholesterol, and prevents arteries from being clogged with plaques and clots. In the context of athletic performance, nitric oxide dilates the smooth muscle layer embedded in the airways, allowing for more efficient transfer of gases to and from the lungs during exercise.

The crux of the matter is clear: shut your mouth and breathe solely through your nose.


Lester started running in 2010 and continues to race whenever opportunities arise. Over the years, he has competed in numerous events ranging from 200m track races to trail ultramarathons. In his spare time, Lester reads widely and goes on microadventures.


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